They will fight for light and root control. This could end up in one or both dying. The best result might be each living together in harmony, but even then they will be smaller and less potent than they would be otherwise, and you would get the same yield as with just one. The best thing you can do is kill one. I would just take scissors to it and clip one at its base. Pulling it up completely by the roots may tear the others roots as well, which would definitely be bad.