Is the hydro-air connection the only heat in the house, or supplemental? If it's supplemental, are you sure you need it? A typical WH can provide maybe 30K btu or so, but it would be of little use for potable water on a cold day. If I understand your setup, you have the boiler of both units, but because they are not the same, flow through each would never be equal. Plus, the temp of the WH outlet normally wouldn't be very hot, and thus the air temp coming out of the hydro-air unit would be low, which isn't the most comfortable, either. If you run the WH hotter, you must install a tempering valve on the outlet to the house for hot to the fixtures.
Depending on your actual heating needs, you might just consider using one tank for heating, and the second one to provide the domestic hot water. Using most WH for heating purposes voids their warranty. Some are setup for this. A better solution might be to install a boiler for the hydro-air coil and use it to heat your water by an indirect WH tank.
WHen trying to extend the hot water volume, often, series works better than parallel. There are some WH that have an internal coil in them designed for space heating which would allow you to separate the potable from the heating water.