and measures each independently. That setup would independently track two battery banks. The negative terminals of the banks are not connected together, each is connected to the high side of its shunt, the low side of the shunts is the common ground.

I agree with Maine Sail, that a good, even small, Gel should crank your small engine well. It could be resistance somewhere in the wiring, or a battery near its end of life (which has increasing internal resistance). The fact that it cranks better when your house battery is paralleled in, suggests the latter (unless the paralleling switch is bypassing a high resistance connection).

I would use a DMV (that's Digital Volt Meter for you non electrical engineering types ), and measure the battery voltage while cranking the engine. You want to measure it right on the battery terminals, not the clamps, or wires, or elsewhere (which might be lower due to resistance in those connections). You will see it drop to 10 or even 9 volts under the heavy draw of the starter. If it drops lower than that then the battery is most likely past its :"best by" date. Then measure it again at the starter terminal to a good ground point on the engine. If there is a big difference between these two measurements, there is high resistance somewhere (which can also be in the ground cables to the engine). It is then fairly easy to use the DVM to track the bad connections down (that will be tomorrow's homework ).

Very occasionally, a starter motor will go bad in a way that draws far too much current. That is a possibility, but seems the least likely.