All depends on the power to your building and which country you are in.
IF you are in a residence in the US, your mains power comes from a center tapped transformer on a power pole or an undergound chamber. Each side of the center tapped winding is 115 volts, nominal. A residential wiring standard is to balance the load on each side of the transformer by powering some rooms from one side and the other rooms from the other side of the transformer. For safety, a single room cannot be supplied with power from both sides.
The same applies to a commercial building, unless the power is 3-phase power. Then there are three separate sources of power to be balanced throughout the building.
For your power line modems to work, they need to be powered from the SAME side of the power transformer.
You will have to try the modems to see if they will work. IF they don't, you have the reason.