Excellent question, without a complete answer. I used the Princess internet but since I was by myself, I only used it to access the onboard Princess 'programs' and check up on my account. I DID NOT try to communicate with anyone back home, nor, try to do business transactions, nor, try to watch episodes of The Love Boat on my phone. I have heard pros and cons from folks trying those activities - mostly negative [except select episodes of 'The Love Boat' should be on your cabin TV -- no need for a small phone screen].
Regarding sharing one phone, nice thought. I (obviously) did not try that option. I see Princess offers a single phone option, or up to four (not sure if a 2-phone package is offered). Supposedly, the multiple option is to easily contact another member of your group when one is at the Spa, and the other is in the casino -- er -- library.
Final thoughts. Initially, the internet program was not very good/reliable. I hear the more recent experiences are much more positive (similar to Medallion - there were growing pains initially). Whatever your internet needs are, I hope Princess is able to give you a pleasurable experience. Keep researching. SOMEBODY has the perfect answer for you. Best wishes, and happy cruising.