RAW doesn't always have the Ramp, so all they would have to do is figure away to put the Smackdown backdrop/fist/screens in front of the parallelogramatron after RAW which wouldn't take long and would save the effort of dismantling the RAW set. I figure they could have it set up in 45 minutes, which would be enough of an intermission to start the show at 9pm SJ time.
Do we even know if they are selling one ticket for both shows or ticketing both shows seperately...that could give them more time to set things up.