I tried Backspice for about 5 weeks with 1 Twiddler3 in my left hand and 1 Twiddler3 in my right. It works for single letters, but I am a developer and I need the things that a normal keyboard has. I use console vim and the Espeakup screen reader, (orca screen reader when I need X but now we have 2 problems) and standards are more important to me than efficiency.
I am switching to TabSpace now. I haven't tried it yet but am about to load the TabSpace twiddler.cfg onto them right now. My high score on http://zty.pe/ as of last night with Backspice was 76. Yesterday is the first time I tried playing with the sound off and my eyes closed except when I'm reading which word I am supposed to shoot next, and I suspect that the performance spike was due to this.
I will wait to see how many problems TabSpace fixes before posting more.