What size wing do you use?
Number of tanks is irrelevant. I use a 40# wing, since I dive dry with thick undergarments, and anything less wouldn't float my rig if I doffed it in the water due to the amount of weight I have to have on my BP. And I dive a single tank setup. If I were following popular opinion here on SB, I'd be using a 30# wing (max), but if I were, I wouldn't be able to doff my rig in the water without seeing it sink. Number of tanks isn't relevant, total configuration is.
How do you set them up? Can you set them up without damaging the manifold or valves? How do you route your hoses? Where do you put the bands?
Those are doubles-specific issues, I'll give you that. But once it's been set up by a competent person, it's not exactly rocket science to remember that setup.
What do you do if you're not trimming out right? What's the proper procedure for first stage or valve failure?
Same as when diving a single tank rig, provided we're staying within rec limits. For the latter, it's "get your buddy's secondary or CESA"
How do you treat them for fills?
Make sure three valves are open instead of one.
There's a lot more to it than just the number of tanks.
My point exactly. Number of tanks is irrelevant. Total gas capacity, weighting and proper practice is.
And I know manufacturers who sell BCs that are good for single and double tank diving.
That's just stupid. But, again, it's not the number of tanks, it's the total buoyancy of the rig.
I also know people who got into the water with double steel 130s filled with nitrox wearing a wetsuit only.
That has nothing to do with number of tanks, and everything to do with total tank capacity and gear config. As long as the diver takes care to stay within NDLs, and has a balanced rig, what has that to do with the number of tanks?
Diving doubles is no different than diving a single tank for recreational dives
Which is exactly what I'm trying to say. Over and over again.
assuming that the diver is squared away when they get into the water.
Well, I guess that's my problem. I assume that the diver is competent to do the diving they're planning to do. I don't automatically assume that the second someone straps two tanks on their back instead of one, all other things equal, they magically transfer to another universe.
Let me put it a different way. A guy walks up to you and says, "I dive with this jacket BC with my this reg, console and an Air 2. I just bought these steel 100 doubles with isolation manifold. Tell me everything I need to know before I jump in the water." Is that a 5 minute conversation?
Less. "Remember your NDLs, follow your computer, and remember to open all three valves. Have you dived with a BP/W before? No? Then we should talk about the difference between a BP/W and a jacket BCD". Now the latter issue is, again, independent of the number of tanks.
it's not the number of tanks. It's total tank capacity, how you set up your system and how you dive.