My recollection is very foggy as I never had a chance to use one of
these things, but there were devices (inexpensive) to do this for use
with printers (at least in one direction) that would not only buffer
up data to be printed, but also do protocol conversions (parallel-to-
serial, serial-to-parallel). If someone had a very old Inmac catalog
from the 80's, I suspect you might find something
there.
There were also devices for sharing terminals on a single port sold
by Inmac and several other companies.
Finding one of these devices themselves would be harder as I suspect
that most have long been discarded as peripheral "junk". In a snap,
almost any old PC with two serial ports could be programmed to do
the same thing.
Cheers,
Chuck