Thanks for the link. If I read their recommendation correctly for two batteries - connect the pos of the two batteries together and the neg of the two batteries together, connect battery 1 to the pos bus bar, connect battery 2 to the neg bus bar (or vice versa). I'd use the same battery for the second one.
El G,
I would think that the buss bar has more current carrying capability than the wire you would put between the two battery positives and two battery negatives.
I suggest that you leave what you have in place... no changes... then, get two more wires, one long enough to go from the neg of the new battery to the neg buss bar, and one long enough to go from the pos of the new battery to the pos buss bar. That is the best way to get the two connected to the buss bars AND have the least amount of wire resistance. They would be wired in parallel which gets you 12V and twice the amps as your current setup.
I added a second battery about 9 months after my first... same make, same model, same battery.... no ill effects because of the slight mismatch created by the age difference!
Looking at your pics, I would suggest making a small wooden U cover ( 3/4" plywood ) with a few cutouts for wires to cover the positive buss bar. Just in case something (wrench, screwdriver, metal anything) gets in the wrong place... Both don't need to be covered.