No problem -- ask questions and learn!
Cruises are priced in "tiers" -- as more categories/staterooms are booked, the fares will increase. For some cruises this increase can be dramatic over time, other cruises not as much from low (early booking) to cruise time. If you are booking onboard, while you get a 10% discount plus OBC, it will be based on the current fares. So if you book onboard just the week prior (to create a B2B), then the cruise may well have increased in fare over time. Now, if you are planning rather last-minute anyway -- say book today for next week, then it might be worth the risk to wait and book a second leg onboard. As long as you keep an eye on the cruise and know it isn't sold out and have an idea of the price to expect, and have a back-up plan in case it suddenly sells out before you get it booked.
Each cruise and each stateroom on a cruise is considered a separate reservation and will have it's own reservation number. You may book B2B at the same time if you wish, or multiple staterooms on the same cruise. But each will have it's own reservation number.
Enjoy your cruises!