That could be a little problematic to have all gateways in the same domain because as far as I understand, Apple provides the domain verification files per platform integrator, so the gateway that provided you with the verification file is your integrator. And I am not sure how Apple treats multi-integrator platforms. Is it possible for you to register as a merchant, decrypt the payload as a merchant and then send the transaction to gateways for authorization? That might be a better solution.
Topic:
App & System Services
SubTopic:
Apple Pay