Well first off, ditch the in game weight because that is ludicrous. Even if a person could swing a 20 pound weapon effectively, it would be a complete waste of energy. A more realistic estimate for the greatswords us midgets are using would be no more than 8-9 lbs, if we're feeling generous. A Scottish claymore for example, was typically just 6-7 lbs. Taking Cub's unique stature into account, a more realistic weight for his hammer would be no more than 15 lbs.
But to answer the question, yes, Vendel and Blade likely could use Cub's weapon and still be effective with it. But it would be highly inefficient and they would burn through energy a lot faster, especially considering the inherent balance problems with club weapons. If they absolutely had no other choice but to use Cub's hammer, their best strategy would be to pulp the first guy they see and then take that dude's weapon.