CM Mobile does not allow you to register two tags to a client however if you use NFC only tags, there is an option to duplicate the tag.
To do this, you can follow these simple steps:
Program the tag for the client using CM Mobile application first.
Download NFC Tools application via the App Store or PlayStore.
Once downloaded open the app and select Other.
Select Option 2 'copy to infinity' (this can be used if the tag needs to be copied multiple times)
Check that the duplicate tags are linked to the correct client in the CM Mobile app.
Erase tag in NFC Tools can be used if you make a mistake and have duplicate tags for the wrong clients.