One boiler, two thermostats

Samaris

We have one boiler and two thermostats. One thermostat controls the first and second floor units that have cast iron radiators while the second controls the garden apartment that has base boards.
When the garden apartment renter turns their thermostat on on the heat doesn’t work for the first and second units. How can that be fixed?

Leon82

Is there a relay box with a priority zone? It may enabled which will hold off the other zones from running