Doesn't matter how much of a boost anything gives ya- your roll will be greatly reduced on the second night, and you'll feel like crap for a week.
Picture a big tank, with a knife valve at the bottom. now imagine serotonin slowly dripping into this tank, and slowly dripping out of the bottom, so the tank stays full, but doesn't overflow.
Now imagine opening that knife valve- all of the serotonin flows out at once, or at least pretty damned quickly. This is what both MDMA and mephedrone do- they're serotonin releasers. there are some other subtleties, none of which work in your favor, but that's the grand picture.
So, we've pulled the valve, we've dumped the tank. Now, serotonin does continue to drip in the top- your brain will slowly replenish the tank. However, if you pull the valve and try to dump the tank again 24 hours later- there's just not a whole lot there, so not much gets dumped. However, you still get all of the side effects of whatever you take, and your system suffers from extreme serotonin depletion for that much longer.
That's why it's recommended to take a long break between uses of releasers- you have to let meaningful amounts of serotonin build up to be dumped. Depending on how strong your reserves going into the experience were, you might have enough left after night one to feel the dose on night two, but it'll be much less intense, dosing higher isn't going to help that much, and the side effects will be nasty.
Seriously, if you have any serotonin left after night one, hang onto it- your gonna want that mid-week.