The "2nd phone number" that's assigned to the watch is to separate it for signal/billing/identification. Otherwise they'd have to figure out which SIM under phone number X they need to deal with for whatever. Don't let technical necessities undermine them giving you what you're looking for.
Look at the bright side, you can give the watch-only number to close friends and family and know that anyone calling just the watch is on a short list.
Look at the bright side, you can give the watch-only number to close friends and family and know that anyone calling just the watch is on a short list.