Let me get this right you have 2 boilers and they want to use one or the other at different times.
If there both closed systems you just need to put in 2 check valves and use 2 pumps. This would be coming off the hot side of the boilers. The return would go to witch ever one was sucking the water. This would work the same on a open loop system.
You could use Water to Water Plate Heat Exchanger. But that would also be fun to describe. But it would not be too hard to draw out. The heat exchanger IMOP is not the way to go.