This is John and Mary. I don’t know why, I just know I’m a Native English speaker and this is what we say. Perhaps because we introduce them each as individuals and use the singular “this”. It’s not They, unless it was a question specifically about their names and other people’s names. “This is Peter. This is John and Mary.” But “these are my parents”.

This is John and marry is correct... These eta ni pravilna

This is John and Mary.

I would say, “This is John and this is Mary.”

They are John and Mary