I have a Large two story house. I have installed two 30 gal tanks downstairs. The cold water comes into the tank from upstairs (aprox 10' drop), and returns upstairs. When I use hot water the cold pipes on the tanks are as hot as the hot water side. I am in Arizona and the cold water is warm during the summer, so I cannot tell if the hot water is getting into the cold lines to the house (I'm sure it is). How do I correct this problem?

Is there a flow control that stops the water from going backwards into the supply line

No. Unless there is one built into the tanks.

Are the two heaters completely separate from one another? Do you have a recirculation line?

Install Check Valve on cold water side. That will prevent hot water entering cold water side...

Chuck asks,
is there a flow control that stops the water from going backwards into the supply line and you answered,
No. Unless there is one built into the tanks. Water heaters do not come with a built in check valve. But by adding a check valve on the cold water supply you are converting your hut water system into a closed system. This means that as the water heats up and expands it will have no place to go but out the pRV ( pressure Relief Valve).
A long with the check valve a expansion tank must also be installed.
On another note. Even with a check valve on the cold water line you will still feel heat in the cold water line for a few feet after the heater's been setting with no draw for awhile. This is caused by convection and as soon as you make a draw the pipe will turrn cold again.
Good luck and thanks for rating my answer. Tom