I currently have 3 (or possibly 4) cadence sensors and 2 speed sensors paired to my Garmin Edge 530. All on different bikes, obviously! I haven't had any problems with this setup.
If I use a different bike, the computer pops up a notification: "Cadence sensor ***** is active, is this correct?" Selecting yes enables the appropriate sensor.
Edit: I had the same setup with my previous Edge 130 and 500 computers as well.