Not sure about muscle gain, but twice a day training definitely worked for me when I was getting lean. A while ago I spent two months doing strength training in the morning 5-6 days a week and 20-30 minutes of conditioning (sprints, sled, ropes, some abdominal work) 2-4 times a week, done about 6 hours after the main session. Ended up the leanest I've ever been.
BUT I did that largely because I had the opportunity to do it without much "overhead" with the second session - I did the conditioning at uni after work, so it took me at most 10 minutes to change and get the equipment ready, then change back and lock the equipment back up. I'd love to be able to do it now, but it would take me about 60-70 minutes on top of the session to get to the gym and back.
In terms of muscle gain, I think a lot will depend on what your sessions look like and whether the compliment each other. If I was doing two lifting sessions a day, I'd probably start with something along the lines of:
AM: Activation (jumps, throws, OL) + strength work (moderate volume and not all out)
PM: Strength work (moderate volume, possibly pushing myself a bit more than in the morning) + assistance work