Ruling on the prayer of one who prayed with a man intending to shorten, then it became clear that the man was completing the prayer

I was traveling to Riyadh, and I was shortening the Zuhr prayer. A man joined me, and it became clear to me after I finished the prayer that he was completing the prayer, and shortening was not required of him as it was for me in the Zuhr prayer. He came in while I was in the final tashahhud, so when I finished the Zuhr prayer, I entered the 'Asr prayer with him. I believed that he was praying shortened, so I entered the 'Asr prayer intending two rak'ahs. Then it became clear that the man was completing the prayer, so I completed the 'Asr prayer with him. So now I am confused; what is the ruling?
This is a mistake on your part. If you repeat your prayer now as four rak'ahs, that is better. Do not make a habit of this. As long as the ma'mum behind you is praying Zuhr, let him make up his prayer. When he has made up his prayer, you and he should pray the 'Asr prayer. The man may be excused for ignorance in this matter, but since he now has only a short time remaining and is asking people at the same time.

The questioner: Should it be made up shortened or completed?The Shaykh: No.
It should be made up completed! Because you entered the prayer as one who is completing it.