They do not treat it as one cruise, however, the second cruise check-in is not required for consecutive cruisers.
each cruise is a separate entity, and your unused onboard credit, Internet minutes, etc. do not transfer from one to the next. Your onboard account will be closed out at the end of the first cruise and you will start a new onboard account for the second cruise.
while you do not need to check-in again, you will need to report to your muster station on the new cruise whether you stay in the same stateroom or not