Don't make the mistake that is often made by DIY'ers and even some electricians. They wire the double-breakers as shown in Figure 7 which causes the passive coupler to be wired from one leg and then right back to the same leg. That doesn't help much. Instead, the passive coupler needs to be wired as shown in Figure 8. Okay, now we can move on to the next logical step.
Lets discuss multiple panel, residential systems.
Figure 9 shows a multiple panel system. This is somewhat simplified since I have seen some electrical distribution systems that looked more like a shopping mall than a house, but this one showing a main panel and 2 sub-panels will illustrate some basic concepts. I want you to pretend those aren't there for a moment. That way I don't have to draw the whole thing over and over again. Sometimes a single coupler-repeater is simply not strong enough to cover the entire facility.