Can I wire multiple thermostats to one circulator?

If you have multiple zones each with their own thermostat and circulator you can use a Honeywell R845A on each pump including the common pump. The common pump in your case would be the circulator between your two boilers. Look at the attached R845A schematic. Note the two sets of load contacts (contacts 3,4 & contacts 5,6). Each circulator would have one of these controllers. The power to the circulator would be switched by contacts 3,4. The second set of contacts (5,6) on every R845A except the one controlling the common circulator would be wired in parallel. This parallel circuit would be wired to the thermostat terminals (T,T) of the common circulator R845A.

The operation would be as follows. When any of the thermostats demand heat, it's R845A controller contacts close, the circulator is powered and the second contacts also close. This second circuit then powers the thermostat input the of the common R845A controller. The paralleling of all the "second" contacts creates a logical OR circuit and the common pump run whenever one or more of the thermostats call for heat.

That should work. Before you rewire just put a light bulb load from neutral to the ZR terminal and see if it works as you desire.