Meaning:

Agree with you. I understand what you feel.

If you say "That makes two of us," you tell someone that you agree with him/her or that you are in the same situation,
or understand his/her situation or feeling.

Example:

I wish I didn't have to take so many final exams this week.
Yeah, that makes two of us.

I have no idea how to use this machine.
That makes two of us.

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