How are Jack and Piggy similar?
It sounds like you feel confident with the differences angle. You picked two great characters for that difference illustration. Jack and Piggy couldn't be more polar opposite. Physically, mentally, and emotionally they are quite different.
With so many differences between the two characters, I understand your difficulty with similarities. I'll try and point out a few.
Both Jack and Piggy recognize and understand the importance of having a leader be a unifying figure for the boys on the island. Jack obviously wants that mantle. Piggy doesn't care for it, and eventually puts his support behind Ralph and tries to help Ralph maintain control. Ralph and Jack have completely different views on how to lead, but Piggy most certainly agrees that the island boys need a leader. In fact, at the first initial vote, Piggy voted for Jack.
Both Piggy and Jack understand the power of Piggy's glasses. Piggy uses his knowledge and starts a fire with his glasses. Jack quickly grasps the power that Piggy yields. And Piggy recognizes his newfound power, too. Piggy knows that as long as he has his glasses, he remains important to the group. Jack knows that too, but Jack sees more importance in the glasses than Piggy himself. That's why Jack eventually takes the glasses from Piggy.
This next one might be a stretch for some readers. It's obvious that Jack is a leader. He may not be kind, but other boys follow his rule. He maintains control. Ralph is losing control of the group almost from day one. I think Piggy has the potential to be every bit the leader that Jack is. Other than his physical appearance, Piggy has what it takes to be a leader. He is definitely intelligent enough for it. Piggy also certainly understands that a leader and a system of rules is nothing if there is not a system of punishments set up to keep rule breakers in line. Ralph doesn't get it. Piggy does. And definitely Jack does. He may rule with an iron fist, but his orders are followed because there is punishment if they aren't. Piggy understands this concept too.