From the way you describe it, I'm guessing you have two circuits (eg heating in two parts of the house) with separate pumps and controls. You want it so that one set of controls (time clock, thermostat, whatever) to turn on it's pump and the boiler - and ditto for the other circuit ?
What you need for that is two relays. Connect the coil of one relay across the circuit for one pump - so the relay turns on when the pump is running. Do the same with the other relay across the other pump. You now have two independent electrical circuits and "dry" contacts that are closed when a circuit is running.
Now connect these dry contacts (ie a normally open contact on the relay) across the boiler run control - both in parallel so either relay can turn on the boiler.
Depending on the circumstances, this can mean that the boiler, and the two circuits can be off different supplies. However, if this is the case, then certain precautions need to be taken for electrical safety. Most notably, you need to ensure you don't cross connect any part of the circuits across the relay isolation (eg sharing neutrals). And any enclosure with more than one supply into it needs to be clearly labelled so that no-one could open it and attempt to work on the circuits without being aware of the need to isolate multiple supplies.
There may be more requirements in a domestic setup - I'm sure someone will chirp up if that is the case.
Ideally, keep everything off one supply with a single point of isolation.
BTW - straying off the electrical side ...
You may also need non-return (check) valves in the plumbing. Without it there is a good chance that when one pump is running, it will push water not just through the boiler, but also in reverse direction around the other loop.
