Batteries Swelling Up, Need Advice

True, you don't need to. But if you want to improve the life of the batteries; prevent one bad battery or string destroying the other; and avoid AGM thermal runaway, you need to isolate parallel connections unless you have some sophisticated battery condition monitoring. If nothing else, the MTBF/MTTF of each battery connected in parallel if half of its normal MTTF - ie, you'd be replacing BOTH batteries twice as often. (Assumption: MTTF is independent breakdown and failure.)

PS - maybe a cap is needed to protect the batteries. If it's a 2x4kW RMS system, I doubt that 2 Kinetics will like it. The thumps could have initiated cell collapse etc. Alternatives are more batteries (I'd suggest 2 extra, though that's a mere guestimate based on casual comments); using flooded cells instead of AGM (probably illegal), increasing individual battery connection resistance, or adding current limiting. Certainly however ascertain if the battery voltage has ever exceeded 14.4V - or whatever the max voltage is for those batteries at whatever temperature - for excessive periods. Short forays above 14.4V may be ok though their damage may accumulate, but 10's of seconds is bad, minutes is suicide. External regulators - especially if attached to the body and not the engine - are quite likely to provide excessive battery voltages. They rely on good grounding, hence multiple redundant engine-chassis/body ground paths. If the charging system has not significantly exceeded the AGMs' max voltage, then it's a cell collapse (thru age or excess current etc) which starts the degradation of all permanently connected parallel batteries. And of course risks explosion or fire for AGMs. (So too for wet cells, but maybe even the inter-cables have been destroyed by acid by that stage!)