First off, ditto what jeromemsn said.
I'm not a plumber but, I tied my coal boiler in parallel with the oil boiler (coal supply into a tee to the oil supply and return into a tee to the return side). Then a circ. in the return line pumping toward the coal boiler. Circ. runs 24/7 controled by an aqua-stat on the coal boiler and a relay that turns on the circ. when coal boiler is up to 130* and also locks out the oil boiler so the two can't run at the same time. The water in both boilers always stay at the same temp. and the zones run as they normally would. Having said all this, one difference between your setup and mine is all my zone circ's. are on the supply side of the boiler, don't think that would make any difference but, as I said I'm not an expert.
Hope this helps a little.