Most 2-up snowmobiles have a longer track than their single seat cousins and have a more robust suspension to handle the weight of two people riding.
If the second person is fairly small, you can get a "Seat Jack" and install that on a one-person sled to have two ride. A dedicated 2-up seat will be quite a bit longer than the one on a single passenger sled, and will not likely fit well, if at all.
The 2 up seat is the same overall length as a single seat, the difference is the 2 up rises straight up from the end of the standard tunnel and the single seat slopes forward. BC is correct about track length and springs. Tunnel extension is grafted onto standard tunnel with 1/4" huck rivets from the factory. I'm sure the rental companies out here had tons of the 2 up seat models, so they are around. Big question will be to find a seat profile that will fit your console. If you go to the Polaris parts website (Snowmobiles OEM Parts - Service & Repair Parts | Official Polaris Snowmobiles Store) you can find what seats will cross over. I would think that the 2000 Sport Touring, as well as any of the Gen II touring models which are 2 up models would fit perfectly.