As mentioned above, issue of connecting a run down battery to fully charged one is overrated.. There will be high current flow for a brief period of time, but it tapers down rather quickly. But you certainly don't want to skimp on the size of the wiring between the two. And you want the sysem to disconnect the parallel contection when the vehicle is not running.

We deal with commercial batteries one of which has an 11,000 amp short circuit capability (1700AH & weighs 6500lbs) and we have had to parallel those into old systems with weak batteries. No problem as long as you know what you are doing.