Since it's a beta2 agonist I highly doubt it would exhibit cross-tolerance with MDMA. But because clenbuterol has stimulant effects on the CNS and the circulatory system it might lead to a bit of overstimulation if MDMA is taken in tandem with it. I don't think the danger is high but it's better to err on the safe side of things with drugs. To be sure, you could indeed stop the clenbuterol a while beforehand, however if you want to take this precaution you will have to stop it a while earlier than 2 days since it has a pretty long half-life (48 hours). So after 48 hours there will still be 50% of the initial dose left in your system. I would stop at least 6 days before your roll, after this time there will still be about 13% left in your body but this is a lot better than 50% and the influence on your roll will be negligible by this time