I have a male gecko I got from someone else. He dropped his tail because he tried to mate with the female he was with and she attacked him. If you're not planning to breed these geckos, you should strongly consider separating them. If he's a juvie, he's probably just becoming sexually mature and breeding season is starting. I just had to separate a pair of C. elegans who were doing well together, but when the breeding season started, the male injured the female when he tried to mate with her. She's recovering from partial tail loss and several bad bites, but they're now separated.
Aliza