I am remodeling my Master Bath. The lighting circuit is shown on the attached drawing (note the red lines on the sketch are actually the white wires in the walls). I haven't installed the actual switch yet (just wire nutting the two wires together).

The ceiling light fixture is a Halo pancake LED fixture with a built in transformer. The medicine cabinet fixtures are IKEA cabinets with LED strip lights each side of the mirrored doors - each cabinet also with a built in transformer.

When the work was going on, only the Halo ceiling light was in use and worked great - the wires for the future medicine cabinets were wire nutted together. I'm nearing the end and tried to install the medicine cabinets as shown in the diagram. As soon as I tie the wires together at the switch location, all the lighting in the medicine cabinets flash on and off in about 1 second intervals and the ceiling fixture will go dim to bright at opposite intervals to the cabinets.

I checked each of the cabinet fixtures independently hooking them up to an outlet...they functioned fine by themselves.