MDMA is better thought of as a modification to the structure of methamphetamine - it is in no way a combination of two different drugs.
It's methamphetamine with the methylenedioxy functional group (in the 3,4- position) - and the result produces is drastically different effects.
You cannot bond two drugs chemically and have them produce effects similar to that of the individual drugs, because the bulk of the second drug attached to the first drug would prevent the first from binding to whatever receptor it binds to to produce it's effects.
You also can't generally create a single drug that will effect multiple different unrelated receptors (ex, combination of cannabinoid, (CB1/2) + psychedelic (5HT2A)), because the things that bind to different classes of receptor are very different (CB activity needs THC-like bicyclic molecule or an aryl-alkyl-indole, while 5HT2A is acted on by phenethylamines, tryptamines, and ergoloids).