I'd like to know how to set 2 or more billing emails in the Billing Details.

Right now, it is not possible to add multiple billing addresses. As a workaround, if you invite the other email address as an owner/billing member, they will receive the billing-related emails too. This is because users who hold Organization Owner or Billing membership will receive billing-related emails (like receipts) as well as anyone listed as “Billing Email” on the Billing Details page.

Please note that these billing-related emails are not configurable in your regular email settings, they cannot be turned off.