Just to confirm and expand what Phil0124 has said, your plan will not work. Taking the most simple case first in your standalone scenario (not Family Setup). An Apple Watch must be paired to an iPhone, it makes no difference whether the watch has cellular function or not. ]
Expanding this slightly, if cellular service is enabled on the Apple Watch it must be linked to a cellular plan which is configured on the paired iPhone. If the iPhone has dual SIM capability it doesn't matter which iPhone account is used, but the corresponding plan must be on the iPhone.
Thinking logically about it, you would have enormous confusion if the watch was receiving uncoordinated messages into a common inbox, some from your iPhone and others from a totally uncoordinated cellular route.
The workaround for emails is to setup your work email as a second mail source on your iPhone and Apple Watch. I do this with my personal iPhone, using Outlook for the work email (I think Apple's Mail app would also work but I want it completely isolated from my personal traffic). This doesn't work for messages to my work phone, but it does mean I can see emails quickly and easily.