I'd never use Clen and ECA together at the same time. They're both stimulants. I've heard from different whom-evers that they hit the same beta receptor but I'm not so sure about this...at the very least, they 'hit' it differently. I'll have to go back and search for info I've since forgotten off the top of my head.
Other than that, I've used Clen and ECA for 2 weeks on and off switching between the two with great success. I believe I did this for about 1-2 months or more and that's w/out any benedryl etc.