i don't think water mechanics work this way, and is easily tested with a BC tank. put in 2 buckets of water, tank doesn't start magically filling to the top.
Your mistake is that there needs to be a space between the 2 portions of liquid! The watering can internal water-exporting-device is specifically designed to create such a gap while pouring water.
Its a very complex and advanced technology concerning stone, bonemeal and the physical properties of concave wooden bowls.
This thing also has a wooden bowl inside: