EE in the UK confirmed to me it is possible but with one small caveat.
I have an old sim only data plan with them and took out a new series 8 contract last Friday, disconnecting the old plan I had.
I purchased an Ultra yesterday as well from a standard retail store and phoned EE, they reverted my disconnection request and will let me have two plans.
However, it is not possible for both plans to be associated with the same iPhone phone number. One Apple Watch plan will need to operate using a standalone phone number that comes as part of the plan.
I am sure that calls and messages still come through from you iPhone when it is in range, but if out of range I assume any calls and messages would only be possible using the Apple Watch specific phone number.