If I check the box that makes a Material ‘Two-Sided’, does this mean that every object that uses this Material becomes twice the poly count? Thanks in advance.
From experience with other engines: essentially, yes.
Well, yea pretty much, although there is still culling when not visible by camera so I would say that it only really counts for complex objects/ bended objects where you can see both sides at the same time.
Ticking Two-Sided Checkbox is not the same as duplicating and inverting faces on the mesh. Briefly, the vertex count will remain the same, pixel shader will run twice.