How can I connect two propane tanks together with a regulator?
You will need to connect the output of the auto change over regulator to the line coming out of the ground.
I am guessing you got the proper 1st stage auto change over.
houses and travel trailers usually have different setups. I had a travel trailer that had the auto setup it was a single stage.
I have built a few homes with propane and they have 2 regulators normally because the tank needs to be away from the home so the tank pressure is reduced by a first stage sent to the house and then reduced again by a 2nd stage.
If you have the wrong size regulator at the tanks the system won’t work well because of restrictions in the plumbing.