I have not done it, but I agree it should be very easy. The coolant lines at the floor are rubber, and there's enough extra slack to cut and add some Tees. Or run them in series. There's so much hot coolant running through the system, you could run both off one loop IMO. I'd probably go heater first, then into the hot water exchanger, so the hot water isn't too hot. The coolant even back there is still very much at engine temp. If you need hotter water, just turn off the blower for a bit.
2000 E350 extended wagon dually
V10 w/ Banks Powerpack, Diablo Predator
Buckstop Outback bumper/grill guard
Reunel rear bumper