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.
In Blender 2.8 double sided normals are always-on by default, but if you want to turn them off (e.g. for export) you can enable Backface Culling in your material's settings:
Note that it's up to the exporter to respect this setting, so this may not work for every format/exporter.
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: