The cage biting might be because they are side by side and want to interact with each other and are frustrated they can't. I've done through the bars intros, and they bite the bars out of frustration then as well (both parties and even all my pups will do it if on opposing sides of the bars). I've also seen stressed gp bite bars (like shortly after bringing home a new gp and it's by itself) or bored gp do it (if they have hay and places to go or hide in, they should be eating it or messing around or hiding and NOT bored biting bars). I thought I had several boy pups initially, and between week 2-3, (after full intros in neutral territory over a couple of days w/o issue) had been letting a pup go into my boar's cage with him. He'd rumble and chase half-heartedly then lay down or rest. They'd be on the bars constantly eventually. Granted they weren't weened yet and it was strictly a "get to know you" session, but they really wanted back in their cage with mom.
My fear with the bar biting is breaking/chipping a tooth. If they refuse to stop, try separating their cages for a couple days and leave a ton of timothy hay in each. If they stop biting the bars, you'll know why. I've also found that every gp I have LOVE hammocks. Try putting one in if you don't already; I made a small mini wooden rectangular table and use a ferret hammock, and attach each end over the top of the wooden "table" so it hangs evenly and is low enough they can get in it but high enough it's off the bedding. Even my newest gp was stressed and rather unhappy after bringing him home, eventually settled into the hammock I'd made for older boar Scottie. Between that and the tunnels, he's either in it or under it or in a tunnel hiding or running around.
