Why am I unable to add two user to the same device online 1 and line 2.

When adding a user to line1 and to line 2 the device is not provision my second request.
Check the user domain and verify that both user information you are trying to assign to line 1 and line 2 are in the same domain.
Check to Verify that both device has the same owner as it is registering to the same phone.

As the owner assigned to multiple device.  Also check the owner to verify that sim. ring is checked and also to have ring all device checked as well.