If you're putting a satellite right next to one of Starlink's, using the same frequency, how'd you communicate with it without the two interfering with each other?
I’m no radio expert but I think this was solved when they managed to handle a hundred thousand connected cellphones in a single stadium.
In other words, it depends on some base stations being significantly farther away from a given device than others. That’s not the case for satellites.
Having more than one satellite above the horizon really only works when both antennas are (highly) directional. Cell phones aren’t.