Gain is a little less than mu of the tube. Might be 15 at the current level used.
Tying two SSMHs back to back as a "balanced" amp is a bad idea in terms of, well, everything. If you want to build a balanced amp, build a balanced amp. But don't build two non balanced amps and think that putting them together makes something better.
Also, conveniently, the 6J6 tube is built to be used as a differential pair.
Edit -- and a shunting pot (Config B below) will allow you to use an off the shelf 2 gang pot, sound better, be better balanced, and will cut the signal by ~6dB.