I think you might be in the wrong code section. A water heater is an appliance and falls under the requirements of Article 422. Article 424 applies to fixed electric space heating equipment.

Even if 424.19 did apply, you're feeding this equipment from more than one source and all disconnects are required to be grouped. Simultaneous disconnect only applies to one power source.

Tom

Few things are harder to put up with than the annoyance of a good example.