Tom,
Here are some shots of my fitting table that I built 16 years ago. I designed it to accomodate one Stanley vise but you could easily put another one of another corner of the table. The only concern I have is how you would join adjacent corners of the same frame. I have never found a real need for a second vise although I do have one more stored in the bottom shelf under the pictured vise. I still use it for joining shadowboxes and I glue and nail each corner consecutively in the one vise.
The bed of the vise is flush with the table top and, if I have an extremely large frame to join, I swivel the vise to the left to clear the underpinner and have at it. The table supports the full frame from the first join to the finished frame.
I also incorporated some drawers for holding nail hole putty, spring clamps, joining books, and other assorted "must haves" for the joining table. I have adjustable shelves to the left of the drawers where I store short cutoffs and other junk.
Here is a shot of the other side of the same table. There are more drawers and adjustable shelves on this side also. You could make deeper shelves to hold longer cutoffs if you wanted to but this layout has worked for me since I built this table. The dimensions of the table are 39" high, the optimum working height for my 5'11" well weathered frame

and the top is over 4' across and over 6' from the wall to the underpinner. I added the shelf for the Euro underpinner when I bought it but everything else was built as a unit out of Baltic birch plywood and oak or pine trim.
(Be sure to incorporate some toe space under your work tables. It makes life alot more pleasant to work without kicking the bottom of the bench all the time. I located a framework of 2x4's recessed under the tables for that purpose.)
The neat thing about all my hand built tables is the fact that, despite their large size, I can move them through any 3'0" exterior door should I need to relocate them. (I have moved these tables 3 times in thet past 16 years and never had a problem getting them out of or into a building through the front doors.)