You will find you have more than two textures to paint with if you use number keys 4-7 in terrain edit mode. Pressing RMB to paint will use the default texture giving you a total of five to paint with
You don't need to over write the original terrain textures, simply name them the same but put into your own folder in the terrainbank, they will show as a selection in test level along with the default.
You can make your own textures default by changing it in style.txt.
Although the textures must have the same naming convention they don't need to follow the format, you could make sediment a cobble path for instance (might have some weird results with raised terrain..lol) but does lead to some interesting possibilities with painting terrain textures.
A funny thing happened on the way to the forum...