Initial Rig Idea

Use of mirrors  An initial idea was to have the two cameras facing each other with two mirrors places at 45o opposite each.  This would then, in theory allow the the images to be taken much closer together than would be allowed with the size of the lenses.
This however is not possible as the variables are too great plus the images themselves would be difficult to control.

Alternate Camera Rig

 

Nikon D90 Rig

Having the cameras side-by-side will enable the lenses to be strapped together.  The shutter release could be used with either rigs.