Hmmm, IMO SCR ( stingray city) wouldn't be worth a 2-tank dive. It's one of those "must do" dives and a really cool experience, but after 45 min to an hour of playing with the rays at 15fsw, you'll be ready to go someplace else.
What I would do, would be to try to find an operator that would do the north wall as the deep dive and SRC as the second. I know some ops do this, because I've done it.
Look for somebody that works out of the Governors Harbor/ Cayman Yacht Club area, that would cut down on the boat ride considerably. I know of a couple dive ops that keep a boat there just for SRC. I think TI Divers has one there, I know Seven Mile Watersports kept their boat there sometimes (or did).
You'll just have to check around and see if you can find someone. Ivan has changed things, not sure about much anymore. Hope I've been a help....