While you can use more than one leak sensor for SHM - If you plan to use SHM to close your valve for plumbing leaks, You can’t set up custom leak SHM apps for leak sensors that wouldn’t necessitate a valve closing. example - sump pump, air conditioner drain, corner of the leaky basement.
For my use case I put all my plumbing related leak sensors in SHM and have the valve close. But for my leak sensors that aren’t plumbing related, You have to use a separate app for that. “Notify me when” is decent for this purpose.