In telephone systems, the device or circuit that interfaces two unidirectional audio signals (one incoming and one outgoing) onto a bidirectional wiring system is called a hybrid. Search for things like telephone schematic and hybrid circuit schematic to see how it is done. Note that what you want to do will be much more difficult because of the total system power level.
While what you are asking about has been around for 130 years, that doesn't make it a beginner's project. What is your skill set for analog circuit design or assembly, what kinds of components do you have access to, where are you located, etc?
Also, how much power is needed at each location? At only 1 W per slave, you are going to have to modulate the tones on top of 100 W of DC power. Sure it can be done, but each slave will need a large heatsink.
Are you sure you are limited to only 2 wires? Three would make a huge difference in the overall complexity and cost of the system.
ak