I'm using Forge 1.x to generate some code but I'm also trying Forge 2.x. Unfortunately it's impossible to use both at the same time. I usually have to remove the .forge folder (I needed to do it from Beta3 to Beta4).
Why not having a .forge1 and .forge2 folder, or .forge/1.x and .forge/2.x. That would be a first start (I'm not sure what else would Forge need)