This is beginning to sound like a "Who's on first?" comedy routine :-)
Sorry, I don't follow your logic.
The EchoCharges and the Duo Charge should be set up to sense any charging voltage on the house batteries, and to bleed some of that off to charge whatever other batteries they're connected to.
If there's a charge on the house batteries from the alternator, the generator, the battery charger, a solar panel, a wind generator, or a hamster turning a little wheel....it doesn't matter...then the EchoCharge and the DuoCharge will handle the task of bleeding off some power to charge their batteries.
Am I missing something here?