I would advise against it, but that's just me.
Last season, we did 3 days per week until school started. Then, we dropped to two days per week. (This was for 5th and 6th graders). I just never felt like we had enough time to work on as many things as I wanted to. I would have LOVED an extra day per week.
We had quite a few first-time players, and the majority of our guys were, well, not exactly superstars. It would've been nice to have some extra practice time.
I guess if you've got an established program and the kids are familiar with the system (and with football in general), then two days per week would work out better. But, for us, it was tough...