To use the sail kit with the Tandem Mirage there is a scupper hole under the forward seat pad in which an adapter fits which accepts the mast of the sail. Therefore you can use the Tandem with a sail kit but only with one person, unless of course you have a smaller person that can sit in the footwell area of the forward seating position (a bit uncomfortable for anybody but a kid or dog).
The above is a quote from Greg Thomas, one of the site administators, about this from another part of the Hobie Forum. So what you heard is apparently correct.
It does seem like not much thought was given to this, but Hobie does not incorporate what to you and me seem like design flaws into their products, based on my limited experience. If I had to guess, the person in the forward seat might have been so close to the mast (and sail) that it might have been somewhat of a safety hazard as the sail whipped back and forth. However, there are mast-mounting kits available for non-Hobie yaks, and you might be able to design a better solution yourself by mounting a mast further forward and slightly higher to provide more clearance.
Check out the recent article in Sea Kayaker magazine on kayak sailing to see what I mean (April, 2005, page 30).