The hammer would have a smaller acceleration than the stake because it (hammer) has a larger mass.
The answer to the second question is not all that clear. If the force exerted by the hammer on the stake 5 kg stake is 10 N, then you might be tempted to think that
acceleration = F/m = 10 N/5 kg = 2 m/s/s.
That is true if the 10 N were the only force acting on the stake. If another such as friction (practical situation) were to be considered, then the net force would not be 10 N, and the acceleration cannot be determined without knowing the magnitude of the force of friction.