This idiom or expression like question means:

I agree.
I really agree.
That is the truth.
That is exactly the truth.

This idiom or expression like sentence means:

I agree.
Me, too.

A. I wish I had a lot of money.
B. That makes two of us.

Also, related:
“That makes one of us,” which means the listener doesn’t agree with the speaker.

A. I wish I could watch TV all the time.
B. That makes one of us.