MDMA will not reduce the effects of LSD but remove the element of fear and bring your imagination into more pleasant areas. This is something that Rick Doblin, the founder of MAPS, describes in the latest Tim Ferriss podcast.
All of the other responses are accurate -- combining the two will augment and possibly lengthen the experience -- but if you're familiar with both substances enough to feel confident in combining them, I've always found that 70mg dose of MDMA and a big glass of water will pull me right out of a trip that's heading south, and pretty quickly, into more glorious places.
But again, disclaimer: this is really only for folks experienced enough to know how to navigate each separate space well, and for those that have confirmed, tested, pure, high quality MDMA.
I highly recommend everyone candy-flip at least once, just to see why they call it candy-flipping (because everything looks like lovey-dovey glowing gumdrops). However, in my experience, the serotonin release from the MDMA tends to overwhelm the LSD, and you pay for it with the inevitable comedown and MDMA hangover the next day instead of the blissful LSD afterglow.