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.

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.

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 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.