The reason for not using one big tank is redundancy. The owner has two choices. Build a system where if a major component fails it is a crises because the building is without heat or hot water. Or, it can be a system where the other equipment can keep things fully operating (or on reduced basis if the boilers are not equally sized) until repairs can be made. If I were the owner I would consider this benefit carefully. Particularly if I was the one they called at 0200 Sunday morning while I was vacationing on a tropical island on the profits from owning apartment buildings.
Short answer to flow metering - yes. And it is workable. You just need meters of the right type. Owner now has to read meters, find out what the fuel costs are (since the gas/electric company reads those meters and bills the owner), calculate apartment share, and bill/collect the money, pay the utility company. This, of course, happens with any design that does not give the apartments their own system. Just one of the factors the owner has to use to choose.
You could go by the amount of water used for DHW. It will probably be a decent approximation. Water temp will vary, so it is not an exact measure. One example - how much cold water comes out before usable hot water in the apt closest to the boiler vs furthest. Keep hot water moving in the circuit to prevent this and it raises everyones costs. It is all about being to say - you are paying your energy costs vs we are sharing and I am going to decide how to do that.
Heating is trickier. Water may flow but the amount of heat extracted in each user may not correlate with the fact that there is water flow. If one apartment has a window open because they like the feel of hot floors it won't allocate properly. If there are zones within an apartment, it is worse. Depending on design with the apartments (size of apartment same of different, multi/single zone, where bypasses are vs variable speed pumps, etc.) It is not necessarily simple. Heating cost is a function of how much heat is removed from the water; not necessarily the amount of water flowing.