I'm helping a sculptor build a really big table and chairs (the 12' table legs are 12x12's). The table is 2 bents with mortise and tenon joints for the "cross beams" and "plates" which join the top of the table leg at the same height.

My problem is that the sculptor requires 2 timbers that run diagonally 2' off the ground (imagine an "X" if looking down from above) between opposite corner table legs. What kind of joinery should I use where these meet the legs? I think a simple mortise and tenon will result in something that can't be assembled.

If the braces that form the x are cut to create a half lap then they should assemble easily and be in the same plane, that is flush surfaces on both sides. Cut the tenons on the ends similar to regular brace tenons and the mortises cut be cut to fit these tenons.
As this is a table and not hooked to any concrete foundation you could just slide the legs onto the pre-assembled x from each side.
I don't see it as a problem, other than the weight of the legs.
You could make a scale model to perfect assembly procedures.
Good luck with your project.
Jim Rogers

I would use a housing w/spline

The main question I have concerns the Plate - Tie connection.

If this were a typical English Tying Joint, where the Plate recieves the Tie, then there would be absolutely no problem with assembly,

my schedule would go something like,

* Braces are assembled (if they in fact meet)
* Posts are slid onto the Braces
* Plates are Dropped onto the Posts
* Ties are Dropped onto the Plates.

And,
if the Plates recieve the Ties, then,
the last two steps would be;

* Plates and Ties are assembled
* Plate Tie Assembly is dropped onto the Post Tops.

If the Diagonals are running in a slope I would agree with Jim, and use Brace Tennons, though, longer than the typical 3" used in many shops,

If the Diagonals are running in a level Plane I would just use a typical Mortise and tennon joint, maybe 5" long, to provide some relish.

With 12" x 12"'s you have more than suffient room to provide any number of housing schemes and, long enough tenons to act in tension.

I really don't think you need a model, but, if there is a question of design, there is nothing wrong with creating a section and an elevation, yes?

Tim Berube