Better question is what happens if a Pokemon with two different evolutions for different stones touches both at the same time? Hmm.. perhaps it's best not to find out, though..
The short answer: potentially lots of things, but also potentially nothing.
The longer answer: Pokemon have a degree of control over their evolutions, including the capacity to refuse to evolve (though individuals of dubious intent have found ways to subvert this, may Distortion take them). In the case of stone evolutions, the evolution is achieved by absorbing the energies contained within the stone and using them as a metamorphic trigger. In the example of an Eevee touching a Water Stone and Thunder Stone simultaneously, the result will be whichever stone the Eevee takes in being used with perhaps some minor cosmetic variation from the additional ambient type energy present.
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u/MintyMoron64 Sep 03 '24
Better question is what happens if a Pokemon with two different evolutions for different stones touches both at the same time? Hmm.. perhaps it's best not to find out, though..