Hey - I had this same debate going round and round in my head. Because of my personal situation, I decided to transfer only 1. While I'd spent a tremendous amount on OE treatments and then DE and didn't want to have to incur an additiona $3-$4k, I knew if I had twins I'd spend a lot more than $4k extra expense just in the first year......think about that. Short term pain, long-term gain.
Also, and I have no idea if there's a medical basis for my thoughts - with a thin lining, will you be able to support 2 embryos until the placenta takes over with it's production of progesterone? I would be very concerned about that but could be way off base in my thinking.
Best of luck to you in whatever you decide. I'm so glad you have a large # of frosties to work with. Best of Luck!!