I ran into this last year also. I had booked with an online TA for Celebrity Vancouver to Hawaii. After doing some research, I decided I might as well see Alaska since I was on that side of the country. I found the Celebrity Seaward to Vancouver the week before which would have been perfect. The TA told me I could, despite my saying I didn't think it was allowed. I called Celebrity and they too said it was ok. People here said it would not be allowed, and then I found copies of letters shared from others who had their trips changed or cancelled by the cruise line for for this B2B itinerary.
I called I called Celebrity again and that rep went to a supervisor who said I could not do the B2B. (I later found out it was flagged in red on the computers after I called, so they didn't have it set up to tell passengers yet)
I eventually got a new TA who also had to do a ton of research to confirm I would be ok with a Royal Seaward to Vancouver, then a Celebrity Vancouver to Hawaii, which seems fine, but same parent company, she wanted to be 100% sure. She had to get a supervisors on both lines to get the confirmation it would be ok, cause the regular reps didn't know.
So I agree, not all reps fully understand the issue. I was very grateful for this board and those who helped break it down for me!