It shouldn't be a problem at all. DH and I did B2B Bahamian cruises and our adult children joined us only for the 2nd cruise. On another of our B2B cruises, a divorced mother and her 2 children (elementary ages) were on the first one and then the father arrived at the end of that cruise to switch places with the mother and cruise with their children on the second one. The father was able to board early with his children and all the rest of us B2B cruisers since his kids were doing the B2B.