50 amps? I'm not sure what size wire even supports that. I'll have to check into this. A double gang box is certainly possible. The problem comes in that the only real location in the garage where a new box can be installed is near the entrance to the garage. I already have installed a 20 amp circuit there, with a single gang box near this location (then metal conduit running around one side of the garage). I believe I can install another single gang box, but a double gang box might be a bit tough. I'll see what I can do, though. (This location is between an oil tank and the door from the garage into the house, and there's not a lot of room between these.)
Also, I do agree with the admonitions to be careful with 240V equipment. In fact, I'll install the 240V line near the "back" of the garage, away from possible water. My garage is known for having a little water come in during certain storms, but the water usually stays right near the garage door. We working to address this issue, but I have to get the family room done soon, as another child is on the way, and we need to prepare for that.