In my team, anyone can review the code made by anyone. That means two seniors can review each other but two juniors too.

What is important exactly in a code review depends on the team. But generally, the same items are reviewed:

* Are team conventions applied?

* Can another understand the new code?

* Is code resolving the issue?

* Can I think about a better code? Something to refactor?

Even two juniors can ask this questions and try to answer them.