I had three zucchini plants that did the same thing this spring. In the case of my plants, I'm pretty sure what happened was that we had some brief cold temperatures early in the season that nipped the growing tip, so the plant grew two stems below the damage. I would think this could happen because of mechanical damage, too, although mine was definitely weather related.

None of my double-stemmed plants produced well. I don't remember if I even got a single squash from any of them. If you still have time, I'd replant. If not, I guess you could try cutting one stem back - after my experience it seems like it would be no great loss.

But maybe others have had better luck?