A 1/8" spacer would just fit between the shafts but this set-up was too unstable to photograph. Tying the interlaced studs tightly together confirmed a 3mm or 1/8" clearance between the two shafts and between shafts and studs. It would not be too difficult to overlap strip laminated plywood housings to cope with this interlocked stud arrangement.
It might make an attractive "skeleton" mounting if the studs could be left visible but covered in alloy tube. The axes shafts could also be covered with a larger tube. Though built-up plywood housings probably makes much more sense from a stiffness point of view.
The large journal bearings have a spherical outer race allowing them to effortlessly self-align. While any linear thrust loads are naturally resisted by the lock between the bearing race and its housing.
I continue to work on drawings in SketchUp to try and improve on the basic design. The ability to rotate and orbit the drawings is very useful for ensuring safe clearances as the mounting axes are moved around.