People often refer to this practice as "double dropping" or "double dunking" and it is usually a recourse taken by those who have developed something of a tolerance to MDMA through frequent use.
Care MUST BE TAKEN that if you're taking more than one of ANY pill that you are sure it doesn't contain harmful ingredients. "SAFE" is not a word that can really be used in any confident sense when discussing double dropping, because by nature you're talking about taking twice the dose of a chemical which in a normal dose is acknowledged by some to be harmful in some ways. (Yes I know this sounds vague, but I'm getting there).
Taking two MDMA pills will cause you to peak quite hard, possibly heat up a fair bit, and the common MDMA upper-level effects (jaw clenching, eye wiggles, massive euphoria etc) will be present. The comedown may also be twice as bad if you suffer after use. Think about things like dehydration, temperature regulation, dehydration, temperature regulation and dehydration. (Get my point?)
Taking two unknown pills is tantamount to gambling... you could be ingesting chemicals that are strongly contraindicated - ie. DMX/MDMA, MDMA/PMA, etc. If you're ever considering double dropping my advice is to ensure that you feel comfortable with a single dose of the pills from that EXACT batch. Double dropping pills you've not yet tried is pretty much gambling also.
Not a safe practice, not something I could recommend in a harm minimisation sense. If you use a tester and make sure you're only taking MDxx pills, and ensure that you take pills that you are familiar with, and make sure to keep yourself hydrated and at a comfortable temperature, then you probably won't die. But don't quote me on that.
BigTrancer
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Load universe into cannon. Aim at brain. Shoot.