Dual monitor allows to have 2 separate screens on 2 monitor.

Stretching one screen over 2 monitors would not work with how the screenbuffer is expected to be layed out in memory so one would need some sort of virtual monitor driver and a lower layer that constantly copies the data to the actual screenbuffers.

Might be possible with some GPU compute but noone has shown any interest in putting in all the work for the rather limited benefit.