Human evolution is a field that's been evolving very fast lately. Even people who graduated only a few years ago are wildly out of date if they haven't kept up.
That said, classical multiregionalism is very, very dead and has been for awhile. Essentially what you're asking about has started to come up lately as a sort of in-Africa multiregionalism. This is a nightmare to model mathematically and so people just couldn't until recently when new fossils made it pretty hard to explain things any other way.
It hasn't fully emerged yet and many people don't really deal with it. Take for instance, this paper. One of the fundamental assumptions is that there was a singular set (or other time limited) of introgression events between archaic and human populations. That's a reasonable assumption in older models (and dramatically simplifies things), but it's possibly violated in an African multiregionalism model.
They claim it's not a problem because of symmetry, a point I'll admit I don't fully understand their explanation for in the supplementary material.
That said, classical multiregionalism is very, very dead and has been for awhile. Essentially what you're asking about has started to come up lately as a sort of in-Africa multiregionalism. This is a nightmare to model mathematically and so people just couldn't until recently when new fossils made it pretty hard to explain things any other way.
It hasn't fully emerged yet and many people don't really deal with it. Take for instance, this paper. One of the fundamental assumptions is that there was a singular set (or other time limited) of introgression events between archaic and human populations. That's a reasonable assumption in older models (and dramatically simplifies things), but it's possibly violated in an African multiregionalism model.
They claim it's not a problem because of symmetry, a point I'll admit I don't fully understand their explanation for in the supplementary material.