Upon Some reflection, how about a thick bar suspended between two or more
jacks?

Hmmmm... I think I am getting into the area where I am just going to have
to try it and see.

First I guess I need to make some jacks with different heads. Flat head,
flat flanged stud head, etc.

Ultimately I am thinking I'll have to go back to the idea of shoehorning two
vises on the machines. Maybe a pair of 4 inch screwless machined out to
hold six inch stock to keep the weight down, and have the compact length.

4 inch mill vise or lockdown vise will not fit inside enclosure without
hitting the doors. Just a hair too long. However my 6 inch screwless
machined out fit just fine. I actually bought a couple 4 inch lockdowns for
those machines. Oops. On the other hand they make great drill press vises.
LOL.

Maybe it would be better to just sacrifice some of my tooling and make some
fixtures with edge clamps.

I definitely do not need another cup of coffee this morning. LOL.