Maybe they had come up with their own sort of time turner and they were experimenting with it, sort of a joke time turner. The fact that its not in their shop in book six could be an indication that it's still got to be fine tuned. But, we can easily enough imagine Fred and George coming up with something like that. It would be like apparition - something worth them pursuing.
I sure hope their joke shop stuff will be in the seventh book. I want to see how "you-no-poo" fits into it!