As far as I know, this hasn't been done. My friend wants to do a live show from his apartment in the evening. Originally he was going to make DVDs to play on my station. But I don't want to be on the air in the evening. And I don't want him hanging out and doing his show at my place.
So- here's what we're going to do- He only lives a couple of blocks from me. He's going to buy the same transmitter, antenna, etc and set it up exactly like mine. We'll both use the same frequency, but never at the same time, and the same "ID's". We are going to pretend to be the same facility. Let them triangulate THAT! I've heard of relay and simulcast, but never two separate facilities pretending to be at the same location. Pretty smart, huh?
Actually that is exactly what Allan Weiner did with a friend of his back when he was really young, before the whole WBCQ gig. He would alternate with another pirate and called it WFSR (Falling Star Network).
Other pirates have been known to do this usually not by choice because in larger cities there may only be one channel that is clear on FM so pirates would have to wait until the other guy was done to begin their broadcast. Same with the popular 6925 shortwave frequency.
I say it's a nice idea if you know the other person and share the channel in a friendly way since it will bring more people in to listen knowing that the frequency is more busy than if it was you alone.
Back in the old days; AM's would share a frequency. In some markets there would three; but two was common. Some AM's also use a different night transmitter. But for undocumented broadcasting in the USA; your ideal is unique and a solution to those who don't want the night guy over every night. I use to lose a lot of sleep with that arrangement; I wish I could have sent them elsewhere.