Is anyone using 2 by 12 lumber to make TBHs.
No. But if I had some laying around for free, I would... I probably wouldn't spend the money on it though.
Well here is a tidbit. I'm helping a new beekeeper build a KTBH in my area. A recent trip to the giant lumber store revealed 2" by 12" by 12' (enough for a 48" hive) is around $18 and a 1" by 12" by 12' is around $20! As of right now it is cheaper to go thicker if you are buying the stock to build a hive.
If you were building a vertical top bar hive it's usually pretty easy to scrounge 1x10 off cuts from construction sites that are long enough for that. Thicker should be better at least as far as thermal qualities. Almost twice as heavy though. ;)
I use the heavy stuff for hive that are stationary. I make my bait hives out of 1 by 12.