He didn’t look back to see which one was behind him, and somehow the choreography was off. He said they had “switched on me” in the clip. This is about poor stagecraft, not confusion about the identities about the individuals.
I saw that also last night. I think this one was an honest mistake - he din't realize where each of them was standing. It certainly doesn't rank up there with last week's declaration that he is running for the United States Senate, being arrested in South Africa, or how "150 million Americans have been murdered by gun violence."
The funniest thing about this was him introducing himself as Joe’s husband.
I assumed that too. But are you ready for something REALLY scary:
https://www.c-span.org/video/?469947-1/joe-biden-addresses-supporters-los-angeles
Spoiler alert: He looked his wife in the eye 10 seconds before that line.