Joe, we did a twin of your setup 2 or 3 years ago at my friends . He and his son shared an OWB for 2 homes.
We replaced the 1 OWB with 2 EFM 520's {one never gets lit} , and that carrys the heating load well.
After about 2 months I convinced him to cut the heat exchangers out . They were not needed or of any advantage .
They love the setup, and wished they did it years ago.