When I play fetch with the dogs, we always use two balls and never switch balls between them. Kali is a fanatic with the Chuck It balls and will play til she drops, if I would let her.

She is the best 'fetcher' around! lol! She always keeps her eye on the ball, in the air and on the ground. She will go after it
the minute the second that you throw it and will run right back and drop it at your feet, happily looking up at you like, Here it is, throw it! Throw it! THROW IT!!!!!!
Buddy will fetch it too, but he also loves to stand and munch on it! lol I can throw Kali 10 balls to Buddy's 1. He'll come and 'bring it' when I ask, but he's not all into the fetch thing like Kali! She's relentless! lol
But like I said, I never switch balls. When Buddy drops his while Kali and I are playing and he 'loses it', I don't let him take Kali's. He can sit there and munch on his ball, let drop it and lose interest for a moment and then suddenly come up to me when I'm ready to throw it for Kali and I'm like, "No Way, Jose! You get your OWN ball.
Find it!" He's really good at it too.
Here is one instance.