If they are in small pots, then you can dump them out onto a
newspaper and (very carefully) separate the two plants. Then plant
each one in its own spot.

Otherwise, the "2 seeds per hole" idea is just to ensure that you'll
get at least one good plant without any blank spots in your garden.
You'll probably have to sacrifice one of them eventually so that the
other can thrive.

I've heard stories of people going ahead and letting both plants
grow together in the same spot--thus, occassionally creating a
mega-plant--but more likely, they will both end up weaker than normal
since they are competing for the same nutrients when they are that
close together.