You don't have to play stuff written or arranged for only saxophones. Look into playing flute duets. There's a plethora of them out there and they are perfectly suitable for saxophones. Like mentioned above, the Bach Two part inventions are great. Also for starters, you might want to try the Voxman duets, fun and easy duets. I just got the Koechlin duets and have played through a couple of them; they're OK.