You should be good with your heat exchanger idea since you have separate runs for each, as long as the units are designed to use a more restrictive metal coaxial air/flue combination. You mentioned 80% efficiency, so I wasn't sure if they were the type of non-condensing power burners which might need the flue to act like a draft flue as described in the link I posted.
90%+ efficiency furnaces condense, therefore exhaust temps are lower and most can vent through plastic pipe. Those burners are designed for that restriction.
True, you only need outside combustion air provisions if you move the units into enclosed or conditioned space.
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"I‘d put my money on the sun and solar energy. What a source of power! I hope we don‘t have to
wait until oil and coal run out before we tackle that." Thomas Edison, 1847 — 1931
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