There is no one-size fits all. If you first figure out the size of the heating plant needed, then choose the brand and model, the installation manual will show typical installation layouts.
Pick a brand, and download the installation manual, study it to get some ideas of what you need/want. Trying to get two apartments comfortable would really require at least two zones and two thermostats. Trying to do it with one would risk a huge energy waste as one would likely be warm, and if they can't control it, they'd open the windows.
I do not know if something like this is available here, but when I lived in Germany, the landlord had a gadget on each radiator that essentially measured the amount of heat produced. The bill was based on that.
An alternative to two zones might be thermostatic valves on each radiator. The system needs to be built with by-passes to allow for the radically varying flow rates. It can get quite complicated, and taking shortcuts can compromise equipment reliability and performance.