This could be solved by replacing the IR source in the 'projector' (actually a point source and a pinhole grid) with one of a different wavelength, and adding appropriate filters to the IR cameras in each Kinect.
Or maybe timing the grid light to be off while the other camera is on and vis-versa. Alternating back and forth quickly like the 3D LCD 'shutter' lenses. This way the grids would not interfere with each other. Don't have to turn off the cameras, just don't use the 3D grid data from those frames where the opposite grid is being used.