For the second time, I practiced tempering the dark chocolate first. It worked! It set nicely at room temperature and had a not-too-bad crack.
So the third time, I was ready. Tempered the dark chocolate, and decided to swirl the PB right in.
I noticed something - the PB cracks easily (see the left piece of chocolate in the picture), so it's best to have mixed in some with dark chocolate, and a little bit on its own to give the swirled look.
And off the chocolate went as Christmas gifts! :)