I’ve made a simple double sided mesh in Maya (an open Cylinder).
Deleted all “History” & “Non-Deformer History” via Edit->Delete All by Type
saved it as .mb and imported it into Unity by dragging the file into the Assets folder.
Unfortunately, the backfaces kinda got lost during the Import…
Do you mean double-sided as material property (this is totally Maya-specific) or a real shelled object (has polys in- and outside, has a wall thickness)? Former won’t work, latter will.
Maya solution: apply a shell modifier or something like that (don’t know the exact way in Maya), or duplicate your cylinder, invert the normals of the second one, and merge them together.
Unity solution: You could write a custom shader/change an existing one to display backfaces. All the built-in ones cull them away for performance reasons.