I have two similar cameras, both 7Ds. Both have Magic Lantern installed and both use their own CF card. Is it safe to borrow a card from the other camera when needed? Or should I uninstall ML from one card before inserting it to the other camera?
There's no real way to "install" Magic Lantern (ML) to a camera. It always remains on the memory card that is used to boot the camera into ML when it is turned on. Put in a card without ML on it and that camera can not be booted with ML, it will use the Canon firmware already in the camera.
As long as the version of ML on your memory card is compatible with the firmware version installed in the camera there should be no issues.