How common is it to have feelings for more than one person?

Not very common but possible. Although the feeling is never equally divided , like you said. In doubt stay with the second person like Johnny once said.

As common as you want it to be.

I remember having intense feelings for this one girl during hs, but like a month later this other girl asked me out and my feelings shifted to her and I wasn't thinking about the other girl. I don't think you can genuinely feel that way for more than one person at a time.

Can you focus on two things in the same time?

So, uncommon.

Switching between tasks is possible. Simultaneously focusing on two things isn't. Brain just be like that.

I can only romantically love one person at a time. I love my friends (girls and guys) but it’s very different, it’s a I care about their wellbeing type love.

I don’t know that it’s common, but I do know it’s possible.

Not sure u can really call it love then

I couldnt be in love with more than one person

I think it all depends on what you view love to be, as love isnt just one stagnant emotion. It evolves and changes, just like people do.

7 billion out of 7 billion people have such feelings.
I mean human beings naturally are not monogamous.