How to penalize students who copy each other but don't realize they have plagiarized?

Learn about your school's policy. Learn about whether your school has an honor code, defines plagiarism in its catalog, or requires mandatory training such as an online orientation. Many schools have specific rules about what kind of punishments are allowed, and these rules may be based on their own interpretations of local laws or their past experiences with lawsuits. For example, my school's policy specifically tells us that we are not allowed to give a student a failing grade in the course for dishonesty, and it also spells out different levels of consequences for a first and second offense.

Look at your own syllabus to see what you said about plagiarism; usually administrators will support you on anything that's laid out clearly in your syllabus.

Don't assume that administrators will support you. Their priorities may be very, very different from yours.

Once you have done all your homework, talk to your dean and propose how you want to handle the issue. Make sure your dean is on board with what you plan to do. Consider "soft" penalties such as forcing the students to meet with the dean.

This is a matter of education. Some students think plagiarism is only copy and pasting from one another. They also think placing a citation is a license to copy. Some also define it as ok if software, eg turnitin, cannot detect it.

In my case I make it clear. If they steal one sentence it is an automatic zero. It is also a zero if they steal the structure of a paragraph or section. These need to be emphasized multiple times until they eject their old ways out of their system.

This isn't just plagarism - it's out and out cheating - and both students are guilty and one of them has kindly produced proof of their guilt. I would escalate to whatever process is usually undertaken for cheating students.

That they apparently have failed to grasp the concept doesn't make them any less guilty of the infringement and it simply doesn't matter whether they "meant to cheat" - they did.

The appropriate way to deal with cheating students is to escalate it to the system for dealing with cheating students. You should trust that this process will take their ignorance into account while (a) being fair to other students and (b) expressing the appropriate level of opprobrium for their actions.