Yes, ideally the amp hours should be the same.
Do you have a battery switch on the front of your trailer?
If no switch: They can be paralleled by connecting the positive posts together, i.e. connecting a cable between the two (+) post on the batteries, then the trailer cable on either positive post. The negative side can either be connected together and grounded to the frame or just ground both batteries to the same frame ground.
If you have a single switch: one cable between batteries pos. posts, one from a positive post to the switch, trailer fed from switch.
That being said, when I add a second battery to my trailer, I intend to replace the single battery switch on my trailer with a dual switch. In this application, the positive cable from each battery is connected to it's own pole on the switch and the trailer cable is connected to another pole. The advantage of this way is you can select to use battery 1, battery 2, or both, and turn off the battery supply simply by turning the switch.
Do not connect the positive to one battery, the ground to other battery, and run a cable between the batteries pos to neg. This will give you 24 volts.