Which phone number is shared by Apple Pay in a dual SIM iPhone?
Neither. Apple Pay uses a billing address and not a mobile phone. Your AppleID can have any phone you want, so either or none would get pre-filled into a form that asks for a phone.

https://support.apple.com/en-us/HT204506
https://support.apple.com/en-us/HT205630

I would get your Apple ID correct with the number you prefer in case that is part of the messages activation process. You can see that in the settings app under iCloud then Name, Phone Numbers, Email. Also check under messages, under send and receive to select one, both or none of your mobile numbers.