The seats can only go on a particular side of the car because of the seatbelt guides.
Under the belt guide is a metal frame that is spot welded to the side of the seat.
Passenger Recliner Seat?
Many people have voiced and interest in having an adjustable passenger seat and bought up the idea of fitting a driver side seat from another smart of opposite drive.
ie: Left hand drive driver seat in the passenger side of a right hand drive vehicle.
The diagrams below will show you that although this is possible, it is not a straight swap.
The dimensions shown are the distances between bolt centres or slider end.
Seats And The Runners
Driver side seat.


The distance between the hole centres from side to side is 41cm.
Passenger side seat.


The distance between the hole centres from side to side is 41cm.
Driver side seat.


The distance between the hole centres from side to side is 41cm.
Passenger side seat.


The distance between the hole centres from side to side is 41cm.


The distance between the hole centres from side to side is 41cm.


The distance between the hole centres from side to side is 41cm.
Conclusion
As you can see, the driver seat on the Fortwo always has the longer runners
and the passenger side on the Fortwo always has the shorter subframe.
If you try to fit a driver side seat to the passenger side, you will be able
to bolt the back down but the sliders will be too long to bolt at the front.
On the Roadster, the swap is straight forward as both sliders are the same.
Although there isn't as much room to adjust the seats backwards in a roadster.
Any Way To Change The Runners?
Of course, have a look
Swapping the runners over will allow what ever 450 fortwo and Roadster seat setup you want.