Oh you obviously mix it up. Nothing in writing is black and white or set in stone. But if you're going to have a character join a conversation, or there are otherwise no good "hints" to let the reader know who's speaking, it doesn't hurt to have the speaker mentioned first instead of after. A few examples:

Bob scratched the back of his neck "Damn these flies are horrible!"

Tina ran a finger along her collar bones. "I know what we could do for fun..."

Fred patted Joseph on the back. "She'l be back soon." 

Sophie clapped her hands together. "Wonderful! We start our journey now!"  

It's less important if there are only two characters in a particular scene. But if you have three or more, you need to work much harder to make sure everything is being said by the correct people.