one boiler and two clocks

Whats the best way to link a boiler to two seperate heating systems. One is an exsisting y-plan system controlled by a clock and stat. The other is an underfloor system controlled by a clock and stat both systems have external controls to the boiler.
My main concern being back feeds down the boiler call wires from both systems, that is when one system calls for the boiler a mains voltage is also sent down the the other switch wire back to the other seperate set of controls. Is it possible to use diodes and resistors inside the boiler to prevent this or is the use of a relay or contactor required with two seperate inputs.
Any advice would be greatly appreciated. Thanks

better asking in electrics

Underfloor section should have been installed with its own pump, manifold, timer and thermostat.
You can not just simply branch off from the flow and lead that into the underlfoor pipe, at least not without upsetting the rest of the system.

No alterations to the boiler should be made.
the manifold should be fed from the main flow, preferably close to the boiler, and "switched" with a zone valve driven by a dedicated timer and theromostat.