The directory structure on the card should be compatible as it conforms to the DCIM format.

But the internal data structure of the .ORF files is different between different cameras.
That's why I had to upgrade to be able to use Lightroom 4.1 to process the OM-D E-M5 files.

Also if you have filenames set to "continue" the camera picks up the last used filename on the card and carries on from there.
Which means you can end up with two cameras with overlapping filename numbering, confusing if you load them all into Lightroom.

I used the file name change feature to add a prefix "1" or "2" on the filenames on each body so I know which OM-D the file comes from.
Simply 1 is the first camera I bought, 2 is the second. That leaves _ prefix as the XZ-1, which hasn't seen the light of day for a while.
The E-P1 is boxed up ready to sell ...