Aside from being somewhat susceptible to intermittent interference on the power line, the scenario you present has an additional potenial challege. Since PLC devices talk over the power lines, the quality and therefore speed of a link between any two devices is heavily dependant on the electrical path between them.
Two PLC devices on the same phase of the same breaker box should have relatively little difficulty talking to one another because the path is as short as the wall to the breaker box and back to the other device. When you have two PLC devices on opposite phases of the same breaker box, communication is most often still possible, but then the signal has to go not only out to the breaker box, but also out to the transformer on the pole so that the signal can come back into the breaker box on the opposite phase and then continue on to the PLC device. In most cases this still works but since there is added complexity as well as added distance in the signal path, this kind of a connection is less reliable and slower than the first example.
In your proposed configuration, my concern is that bridge between the two apartments. Most apartment complexes also have to separately meter electrical usage on a per unit basis, which now adds even more complexity to the situation. Now you have two separate metering systems to traverse as well. These systems might be simple and they might not, depending on how that particular building is wired. Unfortunately the only way to know is to test it.
As far as your bandwidth question, provided that the above issues dont affect your bandwidth significantly, I dont see a problem with printing and internet access. However, I've found VoIP to be highly sensitive to transient interference. I've tried to run VoIP over WiFi before and while it worked well most of the time, there would always be times where calls would drop or get garbled or what have you. All of these problems went away when the connection was wired. In my experience PLC technology is even less stable than WiFi, so I think trying to run VoIP over PLC is asking for trouble.
