Where in the Bible does it tell about two judges ruling at the same time, one evil and the other one good?
Sounds like a trivia question. I suppose some might count Eli as an evil judge (I Samuel 14:8); however, his sin was not in his judgment but in his lack of restraint on his own sons (I Samuel 3:13). Still, Samuel did not really start judging Israel until after Eli's death. The best match that I can think of is Samuel's own sons, though there are more than one of them.
"Now it came to pass when Samuel was old that he made his sons judges over Israel. The name of his firstborn was Joel, and the name of his second, Abijah; they were judges in Beersheba. But his sons did not walk in his ways; they turned aside after dishonest gain, took bribes, and perverted justice" (I Samuel 8:1-3).