The focal length of the lens does not change. It remains a 58mm lens. What changes is the field of view the lens yields on the new, smaller format. The field of view will be similar to that a 58x2 => 116mm lens would yield on 35mm film.
Essentially, you're taking the same photo, with the same focal length, but cropping down due to the smaller sensor format.
To get your crop equivalent with a full frame lens, simply multiply the focal length by the crop factor; so for MFT, just double the focal length. Or, in the case of purchasing a native lens, you halve what you would have used on film. That is, if you want the field of view for portraits that a film 90mm f/2 lens yields on film, then for micro four-thirds you're looking for a 45mm lens (e.g., m.Zuiko 45mm f/1.8).
However. Because the focal length is really 45mm, it won't yield as thin a depth of field as the 90mm would have on the larger format.