To smoke while doing the EC stack? I believe that the EC stack (ephedrine/caffeine) can be stressful on your heart, so would the heart-rate raising properties of weed possibly worsen that? I'm doing the EC stack on and off in 2 week breaks, so its not really hard to just postpone smoking for during those breaks, I just want to know if I need to.
if you're a reasonably healthy individual you will be fine. it's probably not exactly good for you, but i think the ephedra alone makes a far larger negative contribution than what you get from adding weed to the mix.
i did an EC stack last year for probably a lot longer then i should have and smoking never caused any problems. since you said that you're taking the stack during 2 week intervals you shouldn't have any problems at all. from what i've read EC is fairly safe for most healthy people and doesn't cause any issues if its not used over a long term basis.