combining heating zones on boiler
You disconnect one of the end switch wires of the zone you don't want to be able to run the boiler.
When its thermostat calls for heat. It will still open. Just nothing else will happen. It won't start the boiler or circulator. It won't get any heat until one of the other zones calls for heat. Your thermostats are not controlling the boiler, they only control the zone valve. The end switch is what controls the boiler/circulator.
Yes, it will get heat if the upstairs zone calls, or if the other downstairs zone calls for heat. But it can't call for heat itself. It will open and close as the thermostat tells it to. But it becomes a slave to the other zones as far as getting any heat. Only getting heat when one of the other zones are also calling for heat. It does require that you keep the thermostat set to what ever temp you like.
A thermostat is built to carry the load of 2 zone valves. So if you tried to wire 2 zone valve to 1 thermostat. It would burn out the thermostat in relatively short time period.
Just try it. Its much easier, quicker, and cheaper then any other way.