First and foremost, acknowledge and accept that absences will happen. Sure, we'd all like to believe we can have a perfect attendance record, but that's just not terribly likely. As you're planning for someone to miss out on a raid here and there, you may encounter a vital player who swears, "No, boss, no no, I'll never miss."
Hug and cuddle that person, maybe provide some coaching. But don't buy into the idea that they'll never miss. The best case scenario is that they'll place immense stress on themselves to try to have that perfect attendance. But that's not goign to end up healthy.
The worst case scenario is that the other tanks and healers will feel like that person is getting special favors. That can cause angst and misery and end up being divisive among your raid group. So, just deal with the idea up front that people will miss out, and you need to plan ahead to be fair to everyone.
Second, sort out who is most likely going to be the backup. Chances are that your off-tank would roll right over into your main tank position, but then what are you going to do about an off-tank? If you're planning to PUG a tank into Icecrown Citadel, I hope you don't intend to do hard modes. Sure, it can be done, but I'm guessing the average experience will be painful and unsuccessful.
I actually think the idea of having successful backup (due to absence) is probably the most overlooked part of raid composition. If all of your DPS roles are filled by the powerful "pure DPS" classes, then you'll have no hybrids who can swap over to heal or tank. Paladins and druids, of course, get high marks for being able to spec for heals, tanking, and DPS. But warriors are staple tanks, and I've not seen anyone turn a scornful eye at the offer of a warrior tank.
Even beyond the idea of having a couple hybrids around, you still have to have a player who is comfortable in that vital role and whose character has the gear to support that play style. If you're filling in for a main tank who was supposed to handle the damage from Festergut, blue-quality items aren't going to get that job done for you.
The player you select to be backup should probably be somebody with the time to run a few extra Heroics via the Dungeon Finder. I'd like to say "who also has spare Emblems of Frost," but most raids will accept that the backup player's usual role should come first. Nonetheless, a full and complete set of Triumph gear will take a tank or healer a long way.
And don't forget the power of existing content. Trial of Champions has some pretty fat loot. If your backup has the time, help them do a few runs through there to kit out their "off set."
Most importantly, however, make sure that your backup player gets to practice that emergency role. This is going to be your biggest challenge. You'll usually want your main tank or main healer doing that role in progression content, so how the heck do you get a backup player that kind of practice? The easy answer for 25 man raiding groups, of course, is to have your understudies rock out the 10 man version of fights.
Barring the ability to duplicate the fight, however, there are usually equivalent mechanics that you can leverage to create a reasonable facsimile for practice. You just have to get a little creative. If you're trying to practice the "stand on top of me" mechanic for tanking Marrowgar, your tank and off-tank could simply spend a little while running around outside the instance.
Other things take more creativity than that, of course. I've not really found a good replication for the "jump to the other ship" mechanic from the Gunship Battle. But you can get practice with ground targets by grabbing a vehicle in Wintergrasp and getting an idea for how to move the ground target around.
Healers are a little more difficult. When I'm helping a healer get some practice, I mostly run into a Heroic and then pull, pull, pull, faster and faster like my house in on fire. Then, I'll take my pants off. (The first helps with practicing mana management, while the second helps simulate the damage-to-health-pool ratio in raids. And I get to take off my pants.)
The goal here, though, is to try and create opportunities for understudies to get some practice. It shouldn't be a surprise if an unpracticed player picks up a new role and then doesn't perform excellently. I wouldn't lose my mind trying to make sure backup players are just as geared as your main role performers. But, at the same time, if you're going to call on understudies, you have to be prepared beforehand.
Good hunting out there!
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