There was Multi-DOS by Vernon Hester, it worked on
Model I/III/IV
He also made a simple version called V-DOS (I think)
for Game manufacturers.
Percom also had Double-DOS
There were quite a few more, Percom had it's own
incompatible DOS for awhile also.
And then there were the various CP/M flavors for the
TRS-80 like Lifeboat, Lobo, and others.
Al
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