When I'm smoking crystal and weed together, I can usually feel the effects of the weed but no matter how much I smoke it doesn't give me that "stoned" feeling as if I were just smoking it without anything else.
The best way for me to describe it is feeling a little bit "foggy" in the head but the meth always triumphs over the weed and before I know it all I can feel is the speedy feeling after 30 minutes ish of smoking.
My advice, smoke towards the end of the comedown, thats what I do at least with a couple beers by my side (or Xanax if I have some)