50% chance from each parent to pass down their stars, so you'll only need to find a single 2 star boar for breeding

one addition. it is the 50% chance but it seems that after the first parent hittet the next "Baby" (it seems to refer to any breeding in Valheim) has the other parents stats, so that even if your first draw was bad luck the second one will give you the right outcome.
Another hint: It also works with one without even any star and a two-star-parent... every second will be a two-star.

Piggies inherit the star rating of the boar they were born from.

Breeding[valheim.fandom.com]