Here's proper parallel cabling for two batteries. Note that the positive cable attaches to one battery while the negative cable attaches to the other. This keeps both batteries equally charged. Parallel cabling with two 12v batteries essentially gives you you one big 12v battery with double the amp hours.
Price out your battery options both ways (two size 27 or 31s or the 8D, if that's what it is) and add two new battery cables if you go with a pair. You may be better off with one big battery than two smaller ones.