Link out from one of them to link in on the other?
That should work, if I read the schematic correct..
Schematics and info:
http://groupdiy.twin-x.com/albums/userpics/10004/DBX903_Comp.pdf
But only one-way linking will work (master/slave) - and you will probably have to remove the link Y3 on the slave unit. You could then mount a switching jack to the CV in/out terminals, so function is normal with no jack inserted.
Jakob E.