Mount them horizontally.
If you mount as current, it will make the trailer very unbalanced.
Also the tanks design works better with water being able to roll up the side during stopping and acceleration from movement. If you leave them horizontally (as shown) you will notice a lot more back and forth movement when coming to a stop. Also If you leave water in the tank when traveling your trailer will pull/swing more to one side, due to unevenness.
The only way to mount them horizontally, is to still center the larger tank, which means you cant have both parallel to each other.