I agree with Bsiren. I'm feeling a bit lazy, but i figured my post from another thread would do the trick...
While there isn't much official toxicology data on mephedrone i wouldn't recommend taking large amounts of either at the same time for sure. I've used MDMA first, then mephedrone and in reverse order also. So far it seems that doing the MDMA/rolls first, then doing the mephedrone afterwards gives the best effects, but we have only done 3 trials so far.
I don't understand why people would think mephedrone would be a substitute for MDMA. But that is just my experience. Seems like a related, but completely different effect. At least in terms of mental/emotional/empathogenic qualities i would say the two are easy to separate. Mephedrone is fun and grrrrreat for sensual stuff like massage and sex, but it doesn't have the emotional opening qualities that MDMA does. That's the general consensus amongst friends and I.
Mephedrone is more party, less deep verbal communication and MDMA is more emotionally therapeutic/more deep verbal communication, less party. Although i've found that combined with cannabis that mephedrone is more MDMA-like. Now that's a great combo.
I've never done more than about 400 mg of 4-MMC in one sitting though either and i do eat very well so that my body is well supported for a night and then some of hard partying. A few friends and i have been doing really good by starting with 120-150mg insufflated and then boosts of around 40-50mg every 2.5 hours or so.
On average when i take MDMA i try to stay between 120-150mg combination oral ingestion and insufflation ( think 80/40-80/70 split) to start with a possible booster of another 40-60mg around 45-60 minutes in.
Try both of them out and see what you like best. Although be forewarned that, for some people, either MDMA and/or mephedrone can be quite fiendish.
There's more about mephedrone side effects and i recommend reading up more on it.
