As BFL said, yes you can safely parallel those batteries. Not the best solution but for the price of one extra battery than must be 12 volts not a bad idea. Generically a Group24 Deep Cycle is 80 AH and the Group27 is 100 AH giving you 180 AH battery bank with 90 AH being useful (try real hard not to take those batteries below 50%). You should get a couple years of use out of them
When they eventually go replace them with dual 6volts. I just picked up 2 new Trojan T105s for $90/each at a golf cart outlet. This would give you 225 AH which again shouldn't routinely go below 50% but with these batteries you can safely go down to 20% SOC occasional without significantly reducing life. Much more useful, robust and forgiving for boon docking purposes.
