I've got a double width roller door on the garage. I got it fitted, but I did pick up a couple of points about the fitment.
Single doors don't need the balancing roller* that keeps both sides level as they open. As a result, if you have a power opener, it has to be very close to the middle on a single door, but on a double door with a balancing roller, the power opener can be well off centre.
The ends of the tracks that are furthest from the door are kept the correct distance apart by a metal bar running between them. The tracks have to be supported from the roof but they can sway from side to side a bit without causing problems as long as the metal bar keeps them the correct distance apart. If the metal bar is getting in the way, and each rail is independently fixed so that neither can move side to side, the metal bar can be removed.
If you have a power opener, they have terminals where a push button can be attached so that you don't need to find the remote control, or rely on radio waves and batteries to get the door to work.
The opener that I have has a 12V GU5.3 lamp in it that comes on when the door is operating and stays on for 30 seconds or so. It's OK, but it's not a proper garage light. I wired a 12 V relay in parallel with that, and connected the contacts of the relay to bring on the proper garage lights, which I found works well.
*Almost certainly not the correct term. It's a rod that rotates as the door opens, with a pulley at each end. Each pulley has a wire that it rolls up that attaches to one side of the door.
