Even if Goku and Vegeta join forces, they can’t defeat Beerus.
Goku trains with the goal of surpassing Beerus.
Even if Goku and Vegeta join forces, Beerus can’t win.
For your information, Beerus didn’t grow, he just stayed still and the kids couldn’t catch up…
Goku and Vegeta are training for 10,000 years, and are struggling because they couldn’t even defeat Black Frieza in the first place.