Advice on transferring two smaller tanks into one

[dolphinpoint]

Thanks for the advice. I didn't want to use much of the old sand as I read that it can do more harm than good even if you clean it....but a cup of the old sand into the new sand should not be a problem. How long should I let the seeded new tank sit before moving the 2 tanks in along with 50 percent of the water?
Much appreciated!