Two Watch Series 4 attached to same Iphone

I have AT&T, and they told me yes you can have two cellular Series 4 watches attached to the same Iphone. The problem is if watch #1 is in use it cancels out watch #2.

My mom has the cellular one for the fall detection. I wanted to add a second one for me. She uses her's only for the fall detection (and a regular watch). I'd problem probably use mine all day for listening to books at work.

My thought was would having the second watch as WiFi stop it from cancelling out the watch with cellular. So will it work? One cellular, one WiFi? and they won't cancel each other out? (AT&T didn't know and said someone at Apple would). I'm guessing no, but I keep going back that you can be signed in at the same time to an Ipad and Iphone, why not an Iphone and IWatch.

Thanks!