It fires two projectiles but only costs one mini nuke. It's a legendary buff, so it doesn't apply to logic. I had about 70 when I started using it, I just save it for boss-type enemies. The easiest way to get them is to find Super Mutant Suiciders, and snipe them dead before they can explode. They'll leave behind a mini nuke every time.
And yea, the only weapon you can't use in power armor is fist weapons. However, you can upgrade the arms of your power armor to do extra unarmed damage.