Theres nothin in it for him if he's concerned bernie might show him up. If he was confident though, omg he could work the debate so that him and bernie both kind of look good, snd spend the evening shitting on hillary
I don't think bernie has balls enough to go that route. He will try to score political points at trumps expense and trump doesn't need that. The only ones who will care are honestly bernie faithful and they're politically rather irrelevant. If they weren't bernie would be winning.