I'm not aware of valves to transfer fuel from one tank to another. Normally there are a series of values that let you run one or both motors off of either tank or both.
To transfer fuel from one tank to another would require some type of a pump.
If you have diesels the return fuel lines can usually be cross fed back to a different tank. By closing the return line valve on the withdrawal tank and opening the return cross over, you will begin transferring fuel. Just be mindful of the tank level as once it reaches full you will be venting fuel over the side.