Short answer is NO.
The official carrier statement is that if you have 2 cellular watches then you will need 2 Wearable plans. The part about having 2 phone numbers is not really an issue since the carrier's use NumberShare, NumberSync or "insert name here" to bind the watch's assigned phone number to the paired iPhone phone number.
First, the Watch app auto switch option does nothing related to the cellular component. Each watch has it's own eSIM that needs a unique ICCID to be assigned to it during the cellular setup step. If the watch had a physical nano-SIM then you could just swap the SIM like you can with an iPhone and the service would follow the SIM.
Depending on your carrier: you can manually switch between the two watches and reuse a single Wearable data plan, however you will have to go through the complete removal of the watch plan from the iPhone and then complete the Cellular Setup process each time you want to switch watches. It is not as simple as setting the Auto switch on in the Watch app and just putting on the watch you want activated.
Dave