i think the proximity of pianos is extremely important if students are playing on two separate pianos. once, i had to play with someone on the opposite side of the stage. i couldn't tell if i was hearing reverb or if we were together. thankfully, there's always the bobbing head.
seriously, you have to be able to hear each other. if the student is only listening to him/herself - the duet goes nowhere. tempos do not suddenly change unless there is indication. i would set a metronome for a while and make them both stick to the beat. then, gradually - let them play together and stop them when one starts going faster or slower than the beat. or, start tapping the pencil.
one person and then the other can 'take the lead.' this, to me, means being the one to DECIDE to speed up or slow down. this would be much later - when tempos have been established.
i also think that it is only fair to let each person 'shine' in certain portions in recital. so they should highlight where each is going to 'knock it down a notch.'