Is that so? I never said the buckets averaged more than half full.

Yes. You definitely have two buckets.

If there is some parlance in which you can refer to "a bucket of water" and mean "less half a bucket of water" without just being misleading to the point of falsehood then one of those buckets will be an empty bucket and the other bucket has an unspecified amount of water in it. Unless, I suppose, you also failed to mention that after you emptied one bucket into the other bucket you replaced the water with a lemur. Then you would have two buckets, one with water in it and another with a lemur.

Regardless if there is some argument by analogy to be made which requires something which when added to another thing produces just one thing it is best to focus on the water, not the buckets. 'Buckets' is going to rfer to either the actual object or be an ad-hoc unit for water measurement - exactly the thing that the analogy wants to avoid!