This happened to me, and tackleberry has the same problem. Whoever did the dual seat conversion must not have clipped the seat cable in properly. Once clicked in it should not come out. I folled these instructions and it worked. Other ideas were to cut a hole through the seat, if you find that easier :)
just this week we'd a similar situation on the German ST Forum, it is possible, but you'll might need the help of another person around.
- remove panniers
- remove the lower bolts holding the side panels
- lift the rear of the seat on one side to pry out the plastic dowels holding the upper rear of the side panels (this is where the second person might come handy; maybe a tire-bar, large screwdriver or any other kind of lever is needed)
- grab the lower edge of the side panel and bend it outwards to get the clearance to unlatch it first on the rear and then out the lower front fairing, then fiddle it out below the front part of the seat (take care to not brake off the noses clipping into the rubber eyes in the frame
- now remove the two M6 bolts holding the long sideparts of the tailcone
- remove the 4 hex-bolt holding the passenger rails on the cross-frame
- again pry up the rear sides of the seat to loose the two bolts holding the passenger rails underneath it so you can rotate them out to the side (IMO no need to remove them bolts completely)
- pry out the two side parts of the tail-piece
- stand behind taillight, grap tail-cone on the sides and lift it up as possible; maybe needed to remove the 4 rubber grommets in there so you can slide it backwards over the 4 studs poking up from the cross-frame underneath
- pull the tail-piece backwards for aprox 2~3" and then first disconnect the connector of the taillights
Once you get that tail-piece off it should be easy to poke underneath the saddle with a flashlight and unlatch the lock with something like your fingers or a tool like a screwdriver or a piece of welding-wire.