Booking Two Cabins - Best Way to do It?

Going to likely be booking through a first mate. We're really close to my wife's younger brother and his girlfirend. They turn 21 a month apart from each other next year and we're going to take them on a VV cruise for their 21st birthdays. (They're the last in this gen to turn 21!)

My wife and I will need a cabin and then we're going to get one for them, as well (ideally, we'll be close to each other but it won't be a huge deal either way). Is there an easy way to book 2 cabins? Or any sort of deals for this? Thanks!