Hey!
I'm not sure what your specific issue is, but CS:GO itself has a material parameter that allows any kind of texture (known as
$decaltexture) to be displayed on top of the
$basetexture through, as you mentioned, a second UV-channel. This will really
only be displayed correctly in-game (and not even in hammer in some cases, making models look black in the editor but correct when you're in the map). As far as I can tell, in 3DS Max when you import the model from either an
.smd or
.mdl the information for that second UV-channel is lost.
Currently there exists no fix for that and I believe it is Valve only who can add support for this. I even think it was already requested several times, but I don't know if they are willing to provide this support
If you need to recompile an existing Valve model (from any game, really, since the second UV-channel info gets lost regardless upon import), you will be required to reconstruct a second UV-channel and see for yourself how it displays in the viewport inside 3DS Max. What you see is then what you'll get if you export your materials correctly. To do this, however, you will need to tag the 'Export FBX' flag in the WWMT settings for your object, else the second UV-channel will simply not export.
More detailed answer to your question can be given by Shawn, but hopefully this somewhat answered a few of your concerns!
