use a layered shader. You can create a transparency map for one of the textures and then the second texture will show up in those transparent areas. There is a nice tutorial at http://www.mtmckinley.net/
(under "tutorials" then "other")
(I hope it's ok for me to post Mtmckinley's address here, sorry if I shouldn't have)
There can not be Good without Evil, so then it must be good to be Evil sometimes.