Is it meaningful and possible to install two thermostat on one heating boiler and separate the heat
This is what zone valves are for

What you want isn't a standard thermostat that turns the boiler on and off -- what you actually want is to run the circulator and boiler on demand (i.e. they run if any zone is demanding heat) and then to use thermostatically controlled zone valves to turn on and off waterflow to each zone, whether it be a length of fin-tube, a radiator or three, a radiant floor in some room, or an air handler.  With this, you can run all three floors off of the same boiler even -- large buildings routinely are heated using independently zoned hydronic air handlers in each room all run off a single large boiler plant.

(As a sidenote, there are multi-circulator zoning techniques, but a circulator is bigger, heavier, more power-hungry, and more likely to break than a zone valve :)