No one makes a slip-on for the Oil cooleds anymore. Yoshimura used to (I have a set in my garage) If you like the look of duals you can modify the stock system to take the slip-ons for the 93-98 1100's I did this on a 91 using Two Bro's Oval cans on a 91 by welding the proper three bolt flanges onto the stock 91 stainless system. I am selling the 93-98 slip-ons right now for 175.00 and would send the modified 91 header for 75.00 more.
I'd say if your going to spend the money, get rid of the dual pipe system and go with a full header. The cost can't be that much more, it looks better (in my opinion), and make a lot more power than just a slip-on. Not to mention it would be easier to find a full system than to find two slip-ons.