I own two Hollis wings. One is the S38 that I bought to use with my HTS. The other is a 55# doubles wing that I scored on a closeout sale (cost me less than 50%). The doubles wing is huge compared to the S38. I wouldn't want to use the larger wing for singles as the drag would be quite bad. That being said, depending on how much lift you need, the S38 may not be enough for doubles.
One option is to post a "wanted ad" in the marketplace for a wing or two. You may be able to pick up two used wings for the cost of one new one. If you can't find two used wings, maybe you can score one so that you only have to buy one of them new. Dive Gear Express is selling some DR wings on sale right now. You may be able to get two for the price of one that is not on sale.
As for the Air2, the others have said exactly what 99.9% of what everyone else would say (don't use it). To use it with doubles, you will need two first stage regs and a 2nd stage. Assuming you own a first and second stage already, you would still need to buy another first stage. Typically, you won't save a bunch of money buying a first stage without a 2nd stage.
I have begun purchasing the required items to double up my tanks. I already own a singles setup. I was going to buy another Hollis reg to match the one I already have. In the end, I decided not to mainly because the swivel issue with the hose (posted in the Hollis forum) and because I didn't want to have to unscrew and reconfigure my regs when I switched from doubles to singles. Other than changing your hoses around, the only way to dive a single tank with a doubles reg setup is to use an "H" valve. This would work for me at home but not when I travel. I travel frequently so I didn't want that. I opted for separate set of regs that will be designated for doubles use.
I am not affiliated with the shop but Dive Right in Scuba is selling a "doubles package" that includes two of the HOG first stage regs with two of the HOG 2nd stage regs plus an SPG. It seems like a great deal. The regs have gotten good feedback from those that have used them. I actually ordered a first and 2 2nd stages that I am going to use instead of my Hollis reg for singles (again, the hose/swivel issue). I am going to use them this weekend on a night dive.
Hopefully this helps. I realized quickly that doubles are not cheap. I am trying to pick up equipment here and there to help spread out the cost. There are some good deals on used gear so I would keep my eye out for that. Good Luck.