Lots of divers choose Rainbow Reef because they don't charge extra for the dive guide. They are one of the few that do this. Others charge $50 and up for the same service.
As far as a double on the SG with RR? Not typical and on their website, their schedule for next month shows not one double on the SG. When I book a dive trip if possible I book it well in advance and I choose the sites and I know exactly where I'll be diving (weather and conditions not withstanding).
One reason I have been leaning towards the smaller dive ops in the Keys is because they're more flexible and often give the first divers to book the trip the choice of dive site, which can be a double on a wreck. The larger ops such as the ones that are the subject of this thread typically do not have the same flexibility. In fact RR has "Captains Choice" pencilled in as their default dive for many of their upcoming scheduled dives. It's not "Divers Choice".
If you didn't request it then you signed on after another group booked the double wreck dive. RR has an extensive website with their full schedule posted months in advance, I doubt you'll see any double wreck dives on there, like I said there are none for next month.