Use PM to ingest the files to a folder, with the required unique file name. Then ADD the folder into the LR catalog.
Using your suggested file name should be enough, as I doubt you will shoot 2 images on the different cameras at the exact same second, but, just in case, add a sequence number to the file name if you feel it necessary.
Personally, I’d use just the date and a sequence number, as I don’t require a date and time usually. If I need to know the time a photograph was shot I’d use metadata from the camera, same if I wanted to know the camera used.