You would buy one for each cruise, since they close the books at the end of the first cruise and you must settle your account for that portion of your vacation. However, you can ask for any unused bottles from the package on the last night of the cruise to take back to your stateroom, so nothing goes to waste. I don’t know what would happen if you did that and then brought any such back one at a time during the second cruise. Normally, there is a per bottle corkage fee for passenger-supplied bottles of wine.