This must be BS or a regional thing. All the RCA ports I’ve seen in North America are labeled L and R, not L+R and L-R.

What you’re describing is no longer federation but full P2P. From a purely technical point of view, it may work, but the biggest problem will be abuse (spam, excessive resource use, illegal content). When a new instance shows up, how do you know if it’s a spammer or not? And if an instance is blocked by another instance, whose side should you be on?