In a comment, you mentioned how you need to explain this to a sixth grader. With that in mind, I’ll offer this:
In language, sometimes two are treated as one.
Here are some examples:
In those examples, the phrases “ham and cheese” and “Hall and Oates” are treated as a singular unit. The phrase “ham and cheese” could just as well be “ham on pumpernickel” and the concert band could have been “Metallica” or “the London Philharmonic Orchestra". Lexically, I’m merely describing the sandwich and the music group.
I had two sandwiches for lunch today. They were ham and cheese, and peanut butter and jelly.
I have two favorite music groups; they are Hall & Oates and Simon & Garfunkel.
This time, there are two sandwiches, and two music groups, so we need to switch from the singular was to the plural were.
So, back to Magnus and Claudia:
- The phone rang. It was Magnus and Claudia.
- I invited two friends to the party; they are Magnus and Claudia.