Easy way--Either extend the battery platform on the tongue and position your second battery there or modify the tongue area to position the second battery.
Hard way--Mount two batteries inside the storage area. The area around the batteries needs to be enclosed and ventilated. The bottom of the enclosure should be able to withstand acid. Your battery wiring will need to be changed to reach the new location. You may wish to install a slide so that battery maintenance can be performed.
I have a steel box on the tongue of my rig, big enough to hold 2 grp 31 batteries. I chose to install batteries inside the cabin for a few reasons. I went through the steps listed above to do that.