Gardevoir only doubles basic Psychic Energy cards when they are attached to Psychic-Type Pokemon; Double Colorless Energy won't be doubled and would thus be no more use in that combo than a basic Psychic Energy.
Skeptical though I was about the full power of Mewtwo EX, this is why a lot of Mewtwo EX counters, apart from Mewtwo EX itself, fall flat. Mewtwo EX can be played well in just about any deck. Darmanitan with DarMAXitan is a 110 HP Stage 1 Pokemon that needs some form of Energy acceleration to work.
Now Double Colorless Energy works reasonably well on its own, but if you're only getting two coin flips, both must then be heads to get the necessary damage to OHKO Mewtwo EX. If it gets zero or one "heads", Mewtwo EX just OHKOs it back. Plus odds are Mewtwo EX didn't come up until it could at least hit something hard, probably for a OHKO. Prize counts are even but you're out a lot more cards.
Weezing is in a similar boat: Super Explosion doesn't hit hard enough to get around Eviolite without help, but even if you do, you've probably lost a Pokemon before this and thus are merely even on Prize count. Other counters I've heard proposed fall prey to similar issues.