My pattern reads: purl into front and back of the double yarn over - how do I do this???
Purl into one loop of the yo, ptbl on the other loop, or you could purl into the first loop and knit into the other one. There’s a video in the Glossary for ptbl.
The video shows a pfb into a single stitch. Your pfb will be a little easiert since you’ll have more space in the double yo.