I want you first to get your head warped around the notion that you are building a remote supply manifold here with two new zone valves on it - you will also make a new remote return manifold in a similar arrangement when you break into the old return with the additional new return
now if we think about it - and I have often posted that I hate control wiring so often my explanations have that ramification ask your self - all we are adding is what?
well not necessarily adding control we cannot - the current control is maxed ---> we are
interrupting the ONE signal now going to the boiler room controller from the existing tstat ---> by the new flows valve you are installing in that loop
- lets draw this on the bar napkin
the signal from the tstat is now going fromt the old tstat to the controler - powered by the controler low volatage power supply
YOU need a 24 volt power supply at the new remote supply manifold to power the new valves anyway so its now going to power the new and old tstat and they are going to call the new flow valves to open -- when (either of) the flow valve opens - it closes an end switch that normally calls a pump to run - ( check the wiring schematic on the valve )---- bbbb but in your case that switch is now going to re-complete the circuit of the (OLD) tstat to call the (old) flow valve ( back in the boiler room) to supply energy to your new remote supply manifold and send energy past one or the other electro mechanical valves to one or the other load.
Now this is 24 volt so if one or both valve end switches are making the call to the boiler room - it don't make a diff -- the signal from one - the other - or both new valves = will still be a single close and call the system to heat. just that a signal that used to call the zone to attention
There - you just made one supply heat two unique zones- with out a lot of drama - with out running a 24/7 pump - with out running a new whole new supply loop from the boiler. without a new transcontinental control board.
ps -- stain is on tomorrow two coats of clear will get smeared on