The problem with having a room-stat on a landing is that it reads what is a fairly warm area in the house. Stairwells act as chimneys, warm air rises from below, and the thermostat will read that.
The result is that it won't call for heat from the boiler, although the temperature in one of the rooms it controls may be below that required.
The ideal place for a thermostat is on the wall in the downstairs hallway. That's usually the coolest part of an average house, so the stat will call for heat more often. You can then control the temperature in the rooms separately with Thermostatic Radiator Valves (TRVs). That can't happen efficiently if a stat on a landing has told the boiler to stop firing.