If you're looking for dual GPU slots, I can't seem to find one.
Something bothers me though; A mini tower for OC'ing?
I'm not an overclocker myself... But AFAIK, small case and OC just don't get along well. CMIIW anyone. OC is about good cooling system, be it by air or water cooled. That's the main reason why you'll need room for the cooling system. That means big and/or lots of fans. For water cooled, you'll need room for the pipes. I've seen some custom made cases for the compromise between the two factors, but haven't met any after market ones yet.
I'm beginning to see your point here. You're planning to build a mini tower OC'd gaming rig, am I correct? I'm not competent to give advice on OC'ing. The more experienced members might be able to help you on this issue.