I've done this combo a shitload of times and wasted plenty of damn good buds in the process.
The weed doesn't get you higher. Meth seems to totally block its physical effects, just like you can drink a lot of alcohol without getting pissed. Disables the receptors in your head or something.
However: it really fucks up yer brain! Just like when yer normally stoned, you can't keep track of conversations, ideas, sentences, and it makes you paranoid as hell.
Then with meth it makes you doubly paranoid coz your brain has turned to slush but your mouth hasn't! And your mouth will speak whatever is going on in yer head, regardless of whether it's complete insanity, totally irrelevant, or that meant-to-be-secret neverending innerdialogue.
And your fully alert and know you're talking complete shite, but just can't stop, and hafta tell everyone so but luckily they're in the same boat and it's forgotton a moment later as everyone sinks back into their own paranoias and chews their teeth...
Weed doesn't enhance the meth high at all, it detracts from it. Specially in the mental gymnastics department.
Turns you into a kind of special olympic mental gymnast like

But if yer alone and just killing time, then why the fuck not smoke half an ounce.