The only way I could get Samsung pay to work on two watches was to first link watch "A" to the phone. Setup Samsung Pay (SP) on watch "A", adding two debit cards. Then turn off "Bluetooth" on the watch, so the watch does not link with the phone anymore. You might ask why have the 2nd watch at all if it's not linked to the phone, and that is the 64 dollar question I keep asking myself.
Turn off watch "A". Turn on watch "B",proceed to setup SP in the normal way (at this point all the debit cards used for watch "A" will have be deleted). Re-install the debit cards again (this madness---nuts). Now Samsung pay will work on both watches, but one watch no longer links to the phone--no incoming calls, no text messages, no email, but you have SP working.
Samsung are you listening...... This needs to get corrected....