All of this should be available from "early 2019."
Why help each other? Well, BT says it's "the right time" to broaden its distribution of BT Sport. The company has spent a tremendous amount of money on the service, including live TV rights and big-name studio talent. A fresh Premier League bidding war is on the horizon, with technology companies such as Amazon and Facebook reportedly on the prowl. BT will need a war chest to fend them off and its historical rival Sky. Teaming up with the enemy might seem counterproductive — but if it boosts subscriber numbers, and therefore revenue, the trade-off might be worth it for now.
The same argument could be made for Sky. Now TV was designed for cord-cutters who are interested in cheaper streaming services such as Netflix and Amazon Prime Video. BT TV is not its prime competitor (that's a rival for Sky's larger and more expensive satellite TV packages.) At the moment, Sky's priority is to increase Now TV's subscriber base by any means necessary. It needs the platform to have the same popularity and brand recognition as Netflix — and that means being available on every platform imaginable. Buddying up with BT is, for now, a price Sky is willing to pay.