[Stiglr]

Oh! I see now.

Select the surfaces/object and, in the bottom left of the interface, look for the Set Surface type area, specifically the 1S and 2S buttons... click 2S and the object or surface(s) will be 2-sided.... at least in AC3D. I have no idea whether Second Life actually supports 2-sided surfaces (my guess is it does), so I can't guarantee if that will actually WORK in-game.

The other thing to keep in mind is that you may need to control your Hierarchy, which tells Second Life what order to draw/display things in. This will be crucial if, as just one example, your 2-sided objects are "glass" and both need to be transparent, as well as have other objects visible through them. If those transparent items aren't lowest in the heirarchy list, any object below them will not display if between the viewer and that object.

In the Tools Menu, find Hierarchy View and learn how to change the order of objects. Play with it a bit, and get used to how the order can make items appear (and disappear) behind 2-sided objects such as transparent glass. You'll likely also learn that you may need to make glass and transparent objects separate objects (separate from other items they may be in; such as, separate window frame objects from the glass that's in them), so that you can move the glass to the bottom of the heirarchy list.... of course if the glass needs to 'stay with' or 'move with' other objects in the heirarchy, you may have some animation issues to solve.