I use the Amex/Mastercard duo. You only register once for the full account (both cards). There is one bill for both cards, but transactions for each card can be listed individually in the app/online portal. In the transactions tab in the app, just tap the menu icon on the upper right of the transaction list to select the individual card. Transactions for each card are listed individually in statements as well, so you get a pretty good overview of what was done with each card. It would be nice if they had individual overviews of your cash back balances as well. The app/portal are really lacking in that aspect.
Cash back is credited as a statement credit at the end of each anniversary year. If you want to convert the statement credit to actual cash, just use Sonect or Revolut.