A couple of my boys do this when they've decided the hay left in their hay loft isn't quite fresh enough to eat, haven't managed to prevent them doing that yet but I had to separate a different pair at one point and they both started biting the correx and the bars a lot, so I thought it could be a boredom thing. You could try getting them a couple of chew things or just mix their cage up a few times a week and rotate the stuff thats in there, my guys love exploring new stuff in their cage. Other than that I wouldn't know what else to try.
Possibly stick some tape over the nibbled bit but watch this really carefully because when I tried it they just started eating the tape instead. That must be even worse for them than the plastic with all the sticky stuff.
