The NEC requires that each dwelling be served with a minimum of 100 amps. Also I am taking that this is an older building that does not have a separation wall of 1 hour rated or more fire wall. This means that you must install main service disconnects outside by your meter base. Install a ground rod and a PVC conduit up to the meter base connecting the ground rod by grounding electrode conductor to the meter base. Then a nipple out both sides of that double gang meter base to each of the two main serivice rated weatherproof disconnets. Then install either a 4 wire 2 awg SER aluminum nonmetallic sheathed cable or a 4 wire 4 awg copper nonmetallic sheathed cable or conduit containing 4 - 4 awg copper or 4 - 2 awg aluminum conductor inside each conduit to each nonservice rated panel [aka sub panel] inside each apartment.