Yea, I thought of that. It would probably be best in a walk in pen(or just a pen with some height to it) so the pigeons roosts and nests can be up high and keep the ground space clear for the quail. If it's just like 2' high and the nest and roost are on the ground than the pigeons will for sure claim floor space as territory and run other birds out of it. But like I said, shouldn't be a problem with roost and nest up higher. Two waterers and feeders at separate sides of the pen will help, too. Disease could certainly be an issue, but with healthy pigeons and quail I wouldn't worry about it, they don't pose much, if really any threat to each other, especially if you keep things fairly sanitary and run bleach in the water. But, bad things could always happen! I plan on doing the same, so I'm not recommending something that I wouldn't do!