How do I make the faces I am using for the leaves of the tree I am making double sided? Currently only one side appears, while looking at them from a different angle they disappear. The only solution I have read so far is to duplicate the face and then flip the normal so you essentially have two faces back to back, but I'd rather not do this because I am trying to keep my poly count low, these trees are for a game.
For a game, I think that duplicating and flipping the normals is probably the best way to go.
Rendering double sided faces real-time can be very expensive, often more expensive than drawing the extra geometry. Of course, it depends on the engine, so the best thing is to try it and see which works better.
Enable Double sided in Properties > Object Data > Normals:
Press E to stretch the selected in edit mode. Once stretched to the minimum amount possible select all faces and if there are any discrepancies press Ctrl + N to make Normals Consistent.