I know that the queue is excessively backed up, but two games at once is a bit much. I might be able to see it for small games, but seeing as how we can barely generate enough interest to get 40 people for a large game, hosting two at once would effectively kill them both. I personally would sign up for both of them, but I know that not all people would.
Like I said, two small games at once could be an experiment, but that queue isn't as backed up as the large queue.
If we had 50-60 regular mafia players, I would say go ahead. But I remember when I first found the mafia forum that about 3 large games died in signups before TF2 was hosted (about 6 months sooner than choxorn expected IIRC). My point is that we can normally find 35-40 people that want to play mafia at any one time, and some of them will only want to play one at a time.
Just my thoughts.
EDIT: If you wanted more mafia, you could experiment with a 'medium' sized queue, with games having 21-26 (really any number between 26 and 29 for the max), so some small game hosters might add a couple of roles and give the small queue a break, while the smaller large games can get to a faster queue. I usually see all of the large games going for 40, not towards the 21 side of it.
Sorry if this is a bit rambling.