Two buckets are easier because you don't have to lean over away from the bucket.
Work Step by Step
When carrying a heavy load in one hand, the CG of the system (system = you + bucket) shifts in that direction. You must lean away from that hand to shift the CG of the system to be above the support base bounded by your feet. This is discussed on page 143. With two equal buckets, the CG will automatically be centered between your feet.