I've been fiddling with the archmage robes and a standard robe in 3dsmax, and for the most part I've succeeded in combining the two. I have two different textures, one for the archmage's robe and mantle, and the other for the standard robe underneath; so how would I go about combining the two different textures?

What exactly do you mean by "combining textures" ?
If you use a part from the archmage robes and another part from the standard robe, they'll be two different shapes, each having its own texture.

Do you mean combine the meshes into one so that they'll use only one texture?

This is what I figured. If that's the case you'll need something like http://www.gimp.org/downloads/ and something like http://registry.gimp.org/node/70 for gimp in order to be able to import DDS texture files (or convert them into other files if needed). And, well, you'll need to learn how to use the program.