First of all, take one MDMA capsule. I can tell you now from first hand experience taking more than a normal dose (100-130mg MDMA) fucks with your brain.
I took like 500mg last Friday and I'm still feeling the negative effects now. Severe depression, can't hold my thoughts, feel really dizzy and like I'm not in my own body a lot, can't concentrate at all anymore. I'll probably get better in a month but was it worth it for one night of being high? fuck no.
As for the weed, I'd do one capsule of MDMA, see how I felt in 1-2 hours. If I felt like I needed more do one weed capsule. Chances are you'll be feeling just fine off one MDMA capsule. Depending on the dosage anyway.