As you can set up each Apple Watch for your Apple ID individually, you may also need a different activation code for the card you want to add to ApplePay:
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For example, if you add a card to Apple Pay or install an app on one watch, it won't appear automatically on all your watches.
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copied from: Use more than one Apple Watch with your iPhone - Apple Support
Also have a look at this steps:
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If your Apple Watch is lost or stolen, you can:
- Put your Apple Watch in lost mode to suspend the ability to pay from your Apple Watch.
- Sign in to appleid.apple.com using your Apple ID and remove the ability to pay using your credit and debit cards in Wallet.
- In the Devices area, choose the device, then click Remove Items below Apple Pay.
- Call the issuers of your cards.
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copied from: Set up Apple Pay on Apple Watch - Apple Support