Besides the uncertainty of proper flow, which could lead to CO poisoning, the exhaust of your tankless heater will contain a fair bit of water vapor (moisture) as a product of the combustion. All vented tankless water heaters I've seen or installed require that the vent pipe have a downward slope at the exit, so that moisture condensation will flow out of the structure. If you try to vent into the wood stove flue, that moisture will most likely run down into the stove.
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