Interesting concept, I have just done similar with my 2 tanks, one was
cycling, and the other tank had the live rocks toing what the do, the new
tank was taking too long and the level were very bad, I put one live rock in
the tank for 3 days, after that the levels all reached 0. But I think the
transfer of bacteria is actualy going to take much longer then a few hours
or a few days., it seems having the rock there just helps to keep the water
in check but the bacteria levels in the main tank will still have to grow at
their normal rate.