Strip down all the parts from the camera!
Cut the bodies to the wished size. An aluminium body might be more appropriate for cutting.
In general throw out one exposure meter. So you should only use one electronics to control both cameras!
To build the EM3D from two Nikon EMs was a nice project, but it took very
The camera is semi electronic and that was a big problem. The first shutter curtain is released by a mechanical lever, the second curtain by electromagnet. So I had to couple the first curtain mechanically as well as possible to avoid exposure differences. That was hard to build!
Next time I would use a Nikon FG instead of the EM! As far as I know is it nearly identical (mechanically) to the EM but brings features like: full manual exposure control & TTL-flash metering.