In article , "toller" wrote:

It is called a multiwire circuit.
It saves a little wire and can be a bit more efficient on big loads. The
breakers are tied together because they share a neutral, and it is best that
you turn both off it you turn either off. Also, tying them together
prevents you from putting both on the same leg; which is what someone did to
one of mine. (Like 240v circuits, a multiwire uses opposite legs.)

Given that the house is wired with romex (12-2) is there a benefit to
doing it like this? I can see where there might be using 12-3, but if it's
just black/white/ground it seems like the same amount of wire needs to be
run. Maybe 12-3 comes into one of the j-boxes and they use 12-2 from
there?

charles