I see some people here reading two books at once. How on earth do you do that?

I'm interested in reading more than one book at a time, but I feel that I would mix the two books up. Basically, everything I read will get mashed up into one thing.

Does it help to read different genres? Has anyone here read two books at once in the same genre (e.g., 2 fantasy books)? Can anyone give me tips on what I could do?

I want to switch things up every once in a while by reading one book today and another tomorrow. Back and forth, back and forth... And in an ideal world, I'd like to read 2 fantasy books at the same time, but I don't want to read both books halfway and get confused, then have to restart both.

Thanks in advance!