Planning ahead: Email, iTunes, and synchronization
First order of business: who gets it when? If one of you monopolizes the iPad the whole arrangement will rapidly collapse. Alternating days, accommodating commutes or travel -- just clarify up front how much and how often you'll be getting it. Which is good advice in other areas. Ahem.
Decide at the beginning on a few key items: who will be the 'primary' user, or will this be an equal-opportunity iPad? Should everyone's mail, contacts and calendar sync up, or just one person's? While you can synchronize multiple MobileMe accounts to the device, your contacts will be lumped together willy-nilly (calendars should remain discrete, though). If you're a Gmail or Exchange user, realize that full synchronization for those services is mutually exclusive. They both compete for the single ActiveSync config on the iPad, and until OS 4.0 ships in the fall that's going to keep you from push notifications in both accounts. Of course, you can still sync calendars via iCal on your computer, but that's less fun. If you're trying to share multiple household calendars, we recommend BusyCal or BusySync as a quick and inexpensive collaboration tool for families.
Like the iPhone and iPod touch that came before it, the iPad expects to be synchronized with a single iTunes library. While you can subselect content buckets from different libraries (music from here, apps from there) if you have more than one iTunes setup, everything is much simpler if you can pick one computer and one library for synchronization. When buying your new iPad, if you're a multi-machine family, pick a place to sync where you both have full-time access. A laptop that one partner is taking on the road frequently is a bad idea for a sync station, unless that's also where the iPad is going.
In the simplest possible case for iPad sharing, neither one of the spouses has a prior iDevice to take into consideration, and there are no legacy iTunes Store accounts or iTunes libraries to worry about. If that's the situation for you, great -- set up your iPad to sync to a new iTunes library, set up a single shared store account, and roll forward. If it's not the case... well, it gets complicated quickly.
It's relatively easy to share purchased apps between multiple devices; simply copy the Mobile Applications folder or selected apps from one user's library to another, and authorize iTunes using the original purchaser's account. This may bollix up your application updates, however, so it's not a good long-term solution to app purchasing. Best bet is to settle on one partner's iTunes account for iPad app purchasing going forward.
Getting all the non-app content consolidated into a single sync location is a bit more straightforward. If you have multiple users with iTunes libraries on the same computer, you can share music and media between users using this approach; there are more complex ways, and tools to be used if you want to automate the process of managing multiple libraries. If, however, you've got different iTunes libraries living on different computers, things are a lot easier than they used to be.
Share and share alike: Leverage the power of Home Sharing
As of iTunes 9, the chore of coordinating and synchronizing iTunes libraries across multiple machines was radically simplified. Home Sharing allows machines on the same LAN to automatically update each other with the latest content purchased from the iTunes & App Stores. Setup is fairly easy; just activate the feature under the Advanced menu in iTunes, making sure you register all the machines in question with the same iTunes account (preferably the one you decided on as your master account for purchases). You can choose automatic or manual updates for apps and other media from the Stores; for content you added yourself from other sources, simply use the "Show Items Not In My Library" pop-up menu choice at the bottom of the Home Sharing screen in iTunes, and grab everything you want to include in the master library.
The power of Home Sharing really kicks in when you grapple with a second iDevice, synced to a different home computer. As long as both users are logged in with the same iTunes store account and the machines are on the same network, any application purchases made on a spouse's iPhone or iPod touch are easily available for the iPad sync station. Nothing helps marital harmony like knowing your impulse purchase of Doodle Jump will not be AWOL when you go to snag the iPad for your commute.
Note that Home Sharing does require all the participating computers to be on the same network and to have iTunes running simultaneously when they check in for unsynchronized items. It's not a solution for sync to office computers or the in-laws' PC (unless you're planning to rig up a private VPN configuration).
Keeping order: Application sorting and priority
My recommendation for picking a single sync station may seem a little bit restrictive. If you're trading off iPad use on a daily or weekly basis, why not synchronize independently to each partner's computer and iTunes library? iPad sync is noticeably faster than on older iGear, and if you don't have a slew of big apps or large media files it wouldn't take too long. Still a bad idea though: each sync to a different library will reset your app sorting back to the alphabetical default, and your carefully crafted home screen layouts will be for naught.
On that front, be sure to communicate with your partner about high-priority app positioning (first two screens, the app dock) rather than simply moving things to where they are convenient for you. It may not seem important that Goodreader or Air Video own a spot in the dock, but unless you know your spouse's use patterns you can't say why they might want an app where it is. The iPad's personalization process includes moving apps where you want them.
Likewise, don't adjust application preferences or other settings on the device without at least making note of what you changed. It's never pleasant to get an irate email or IM from your spouse saying "What did you do to the iPad?" (Note: At some point you are going to get an irate email, IM, direct message or phone call asking what you did to the iPad. It is fundamentally unavoidable.)
Setec Astronomy: Finding room for privacy
Speaking of awkward conversations, the iPad's native state of single-user-ness means that there are few if any intra-device privacy controls, and spousal trust is all-important. Unlike separate accounts on a Mac or Windows machine, where there is some segregation of personal data, when you're sharing your partner's iPad all the emails, Tweets, Facebook and Foursquare updates from the device are an open book once it gets handed off.
While you can use a Twitter app with multiple account support, there's still no way to block other users on the device from browsing your past conversations. IM messages, RSS subscriptions, videos, browser auto-complete, bookmarks... the list could go on for a while. Obviously, honest and frank expectations about personal data and privacy should be shared between the two of you -- what's truly off-limits, what is shareable or browseable. If you really truly can't trust your partner not to dig into your stuff, consider using web clients for mail or other services. Or get a 2nd iPad.
Speak and be heard: Communication is the key
All of these suggestions can be distilled down to the Golden Rule of relationship reckoning: don't clam up. If you're uncomfortable with the way that you and your spouse have worked out your iPad share, be sure to have the conversation rather than letting the matter simmer for an extended period of time. If everything is working out for your arrangement, congratulations! If not... well, see the suggestion above.
Are you sharing an iPad with your spouse or (heaven forbid!) your children? Let us know what you've learned.