I had a cattledog who would play fetch all day if you let him. If you just had one ball, he'd trap it under his paws until he caught his breath a little and you'd never get him tired out. But if you had two, you could throw/hit one when he was most of the way back and he'd drop the one he had and chase the other. Do that for about ten minutes and he'd finally get winded enough you could do something else for a bit. When he'd recovered again a ball would land in your lap or by your feet. If you didn't notice, he had a piercing bark that he used to good effect.