Thermostats

So many questions-so little time!
A conventional thermostat is a simple switch, so it cannot average the temperature mesured in two different areas. There are more advanced thermostats which can take the average from several different locations to control the system.
Use of these more sophisticated thermostats requires the system to be in perfect balance, with the radiators receiving steam all at the same time. This is achieved with generous main venting.
Tell us what you are trying to accomplish, and we will help with a solution.—NBC

You can put two thermostats in parallel, quite easily, if they re conventional two wire thermostats. If they are fancier three wire thermostats, then it's a little more complicated.

However.

That will not solve your problem of imbalance in heating between your two northside apartments and the rest of the building.

Those two apartments lose heat faster than the others -- wind, and no sun. There are ways to attack this problem, but two thermostats together isn't one of them.

What is your heating system? Steam? Hot water? Forced air?

How is the insulation in the building? How tight are the windows and doors?

If you parallel the two thermostats, yes -- the boiler will run whenever either one is calling for heat.

This will not solve the temperature imbalance between the various spaces.

If -- and only if -- the steam mains in the basement are properly vented, you may be able to improve the balance by adjusting the vents on the radiators (assuming this is one pipe steam).