2 Thermostats one furnace
Is there a way to have one zone at a time calling for heat?


yes

maybe with a newer style boiler controller like a hydrostat 3250 or definitely with a zone control such as Taco SR503-4 Switching Relay

at first thought you could wire one of the zones as the domestic hot water priority on the boiler controller provided it has a priority zone connection, which is typically where your boiler heats an indirect hot water tank (for your shower and all your faucets) in addition to heating zones... so whenever the device on the DHW priority calls for heat it gets priority and all the other heat zones if they were calling for heat have to wait for whatever is on that priority zone stops calling for heat.  You would just choose whichever zone of your two would be the priority, and if you have a domestic hot water indirect tank then put that on a different regular zone connection.

A zone control like the TACO I mentioned can do it


  Taco Switching Relays combine clearly labeled PC board layouts with advanced features that allow for total system customization. Whether using basic priority switching or the advanced features of the -EXP controls, the job is made easy


you can read hydrostat hydrolevel manual here to understand its capabilities: https://hydrolevel.com/fuel-smart-hydrostat-3200-plus3250-plus/