How to get my dogs to fetch other objects

I have two dogs and one is a master of fetch with a ball and the other is good but couldn't keep up with the other and would just jump and bark. Even if I threw one ball and then immediately threw another one, the slightly slower dog wouldn't go for it. So, I got a Kong tumbler and my faster dog is obsessed with it. Great. Now, I throw the Tumbler for my faster dog and then immediately throw the ball for my slower dog. All was well. However, I've run into problems.
1. My dog with the Tumbler gets much more of a work out and I need them to be equally exercised.
2. Even if I have two Tumblers and throw them at the same time or one after the other, the slower dog could care less.
3. The slower dog randomly chooses when she's done playing because, in the end, running off with the ball and chewing it into destruction is more rewarding than anything else I can offer her including food. She doesn't manage to get away with the ball before I call the game over all the time but enough in that it's a problem.
4. When I bring out a different ball (one that can't be destroyed), she won't play.

What can I do to get my slower dog to either enjoy the Tumbler like my faster dog or at least play fetch with a different ball that won't be rewarding for her to run off and chew to death?

In my experience dogs either enjoy playing fetch and would play all day or they don't enjoy it and nothing will make them. I suggest you find an alternative way to exercise your dog. Try running around yourself and let the dog chase you.

My two are similar. The dog which actually has the better retrieve is now a little slower than he used to be, and he was beginning to lose interest rather than compete against the faster dog. For a while throwing for the steadier dog while the first one was engaged on her out lap worked, but she then started leaving her own ball and interfering with his home run.
By this stage it has become less a game than a competition for status, which the same dog is set to lose every time. I now only play retrieve one dog at a time. This gives them an equal chance of success and approval, and the same amount of exercise.

They both enjoy fetch... a lot. They've just decided that chewing up the ball is more rewarding after a certain amount of time. So, rewarding that they will end fetch earlier than normal so I don't have a chance to take the balls away from them. I really don't want to encourage chasing. Plus, my knee can't handle more than just walking.

Yeah, my slower dog won't play unless the other dog is playing too.

Have you tried any other games with the slower dog?

Or even a different type of ball, maybe a soft latex ball or one with a squeaker.