Talk about jerry rigging... eek... Guys, that sounds a little, um... out there?
If I'm getting this right, there's a 6V (good, bad, indifferent) in the equation that the inverter never sees?
The surplus juice dissipating into the 'donor' battery as it just goes into runaway, dumping it as heat?
Never done such a thing in practice... not gonna knock it over definitively either, but it sounds like just plain old bad practice... ?
Steve