U-verse only allows 4 streams, so if you had them all running at the same time you will run into problems. But if you stayed with in the 4 stream quota then you would be alright. I'm not sure if accessing the multi-room DVR counts as one of your streams or not. I'm thinking it does not but I'm not a hundred percent sure since I only have 3 boxes. Rarely we ever have more than 1 going at a time.

Any heavy users out there that can answer this one? 

I guess I missed the whole thing about splitting the coax for two TVs. I agree with Sonic Dave, as usually. This might have to be a trial and error type thing. But you should be able to split it coming out of the box to 2 tv's. I don't do this personally but my buddy has this set up for TV's he has that are on opposite sides of his wall and just uses 1 box to control them both.