Our first Hobie Tandem (# 0402) was an '01 model built in '00. There was no kayak sailing capability then and I'm pretty sure neither adapter would work on it. I believe this information came from the Hobie plant. The seats were riveted on and not removable, possibly having something to do with this. The hull bottom had sponsons (flared hull) like the Classic (changed the following year??) and the boat had a smaller 450 lb. capacity (instead of 550 lb.). See picture below:
So I'm reasonably confident that NOT ALL Tandems can accept a sail. Be careful what you get -- I would not consider anything older than an '02 model to have any sailing capability; maybe newer. By '04 though, both adapters were available.
I think Dog may be right about the adapters fitting into the front seat scuppers, but am not sure. I can see no other place to put it and there is no mid-hull access on the older models. Without a front passenger though, you definitely need ballast to get any decent sailing at all.