Exactly, I ran in to this situation on my most recently booked cruise. The two segments were already on sale, but the combined one wasn't. I wanted to get two credits out of it anyway, so it worked for me. I have one booking number, 2 Cruise ID numbers. I have double Oceania Club credits, $500 each segment as well as $250 SBC per segment for booking onboard.
The real bonus though is that I get TWO bottles of that delicious Oceania Club wine, one for each segment.