With the tennis ball, I too use two balls and a chuck-it. When Kayla returns with the first, I have her drop it with an out command. Sometimes we do it with a 180 degree reverse direction and I call for her to drop the first as she sprints by. I do not toss the second until she releases the first.
That being said, I only use the tennis ball for play. For training, I use various tug toys, treats and praise for awarding/motivation, and praise/clicker for marking. With obedience/rally exercises, I use treats or a tug toy that I hold on to as I rarely want her going away from me. In exercises like agility, I will use toss toys/tugs when I want her driving forward and away from me in areas like driving through the end of the weave poles, or over obstacles such as jumps or out of the tunnels. With the tug toys, she brings it back because the main motivation is actually tugging and she can not do that without a partner.
In the case you put out there, you’ve got some great examples from folks on how to train to what you want. And you need to determine if one method of award works better that another. I've found with Kayla, balls,tugs, treats, praise are pretty interchangeable as I've always mixed them up.
I don’t want to dismiss Ollie’s warning about the tennis balls. To add to that, others here on the board have pointed out that as a ball accumulates dirt, it acts like sandpaper against the teeth as well. That being said, I only allow use when we’re out playing and they get put away as soon as we get back to the car/home. You will have to decide if you want to continue using tennis balls or find/use something different or in addition to the tennis ball.