Mixing stock speakers with other speakers?

I want to add some new speakers to my car, and I don't know the name of them, but all I know is that they're by Boston Acoustics, and they have midb and tweeters. I was wondering if I could add those along with my stock speakers? Or should I replace the stock speakers with them? I want to keep the stock speakers but idk if mixing stock with aftermarket speakers will kind of...idk, "clash" and sound crappy.