Another vote for an energy audit. Any house that needs two 75,000 BTU furnaces is monstrously huge, terribly inefficient, or has severely oversized furnaces. I suggest putting time and effort into energy efficiency until the water heater and furnaces die a natural death. Then replace them with energy efficient properly sized furnaces.
For comparison, my new energy efficient house needs only 13,000 BTUH total heat when it's -30 deg F outside and 70 deg F inside.
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