What You have is a fine way to introduce yourself to Solar ,many of Us began with less.
If You have 2 x 12 Volt Batteries they are connected to each other in Parralell ...Positive to Positive and Negative to Negative. The Lead Battery is the one that has the Positive feed attached to the Fuse Panel!
If You have 2 x 6 Volt Batteries they are connected to each other in Series! Positive on One to the Negative on the Other. This results in One Large 12 Volt Battery. In this case the Lead Battery would be the One whose Positive Terminal is connected to the Fuse Panel.
In either case be sure that The Wire from the Solar array is connected + Positive to +. And the Legative is connected to the - Negative or Ground to the Frame!
May I suggest that You Google Search " The 12 Volt Side of Life"