If you have 2 50 gal tanks this is what I would do.. I would transfer the tank that I was buying with the sand at the bottom (depending on the size of the tank of course) drain all teh water from that tank and put them into rubbermaid containers since this water is already establed it will help when your puting the tank back together. Leave about an inch of water above the sand to keep it live and transport it to you home. Split the rock and fish into your 2 50 gal tanks and fill up the tank when your filling it up place a garbage bag over the sand bed because when your adding the water back into the tank this will not cause the sand bed to stir up saving a huge spike and lessoning the time it will take for it to cycle. After a couple of days and a few water changes you should be able to put your livestock and rock back into your purchased tank.. Few days if not a week of headache but shoule be worth it in the long run
Good luck. If you need to borrow a few heaters and a few power heads let me know as I have a few extra kickin around that you can borrow