Assume you mean propane tanks? Not sure about your exact set up but there is usually a valve at the top of the tanks if they are located on the trailer or somewhere in the propane tank compartments (on my trailer there is a valve above the passenger tank in the compartment) that moves 180 degrees. Pointing forward is the passenger tank, back is the driver side and in the middle is both tanks simultaneously.
I recommend against both tanks simultaneously because when you run out there is no reserve. I drain one completely then camp several more days in the other and then fill both at the same time. Most places have a minimum charge so i like to exceed the minimum as that reduces the price per gallon.