To save Drawcalls, I used the PNG format for opaque and transparent / half-transparent textures, and placed both texture types in a texture map (see image below). Since the map also has no background but is fully transparent, SU applies the texture on both sides (on the outside and inside of a surface). This does not happen with JPG textures.
It would be very nice and especially helpful if you could change this in a future version so that even such textures on the selected area would be applied only one-sided. This would help save many drawcalls and work.