Theory: Goku and Vegeta already managed to hit Whis, but...

[nineko]

... he rewound time.

I think they might need to hit him several times over the course of three consecutive minutes to make it apparent that they did so, otherwise he can just keep on hiding the evidence and pretending that they didn't.

What do you think? It's a stretch, but it might be possible.

[theherodjl]

I don't think so. Whis would probably be way too proud of them even being able to slightly scuff him, to want to trivially negate their success like that. Goku's Perfected UI got easily blitzed by Whis' UI so I don't imagine that any of he or Vegeta's lesser forms have been capable of landing a hit thus far. There's a scene from the anime in which Goku was able to bite the hand of Whis during training, but Whis either allowed that to happen or his UI didn't factor that action in as a fighting move; it doesn't really matter as Whis didn't rewind time for it anyways.

[ChronoTwigger]

The moment Goku will hit Whis, it mean he surpassed Whis UI, and that mean he can hit Whis repeadetly at will. Further trainings will be of no use, if not to turn Whis into a punching ball for no extra avail.
Despite this, what Whis said at the beginning of Granolah arc set up the matter. Goku UI is a ridiculous fraction of actual Whis UI, more far than kid Goku was to SSJ. We'll probably never see Goku on par of Whis.

It's more logic anyway to figure if Goku is still subpar of Beerus, considering what happened in BoG and how better have become since then.

[nineko]

While I agree that Goku and Vegeta will (probably) never be on par with Whis one-on-one, I was mainly referring to the possibility that they might have managed to hit him through their combined actions during one of their two-on-one sparring sessions.

I know it's unlikely, but still, it would be a nice plot twist, "oh yeah you already hit me two years ago but I rewound time, trololo".

You see, I'm about to write a book whose main character has a similar ability (actually inspired by the rerecording features used in tool-assisted speedruns), that's why I'm considering all possible scenarios.