Warped Double 2 x 6 Tread
The biggest problem I can think of when 2 x 6 stair treads like the ones in the picture above start to warp or twists, will be the uneven surface it provides for the step itself. I've said it before and any time a stairway becomes an obstacle course, because something is making it difficult to walk up and down, then this should be your first clue that you actually have a problem.
If you're planning on designing a stairway with double 2 x 6 then it
wouldn't be a bad idea to select a premium grade lumber without using any
center cut materials. You might even consider using pre-manufactured
engineered materials like those used for outdoor decking.
Either way, it wouldn't be a bad idea to maintain a set of exterior stairs like these on a regular basis and this would include painting them and replacing any warped or damaged 2 x 6 wood stair treads.