I can't remember seeing one, but you could put them both on the same side by mounting a plate to the bearings and mounting both Z axes to that (instead of directly to the bearings). The trade offs would be that this way would require a larger y axis travel vs back to back requiring a longer x axis for both spindles to cover a given area. For back to back one spindle would also be farther from the y axis bearings. With the side to side mounting you could also more optimally position the spindles between the x axis bearings.
CNC mill build thread: http://www.cnczone.com/forums/vertical_mill_lathe_project_log/110305-gantry_mill.html