Both domains are controlled by the same account, one is a placeholder domain, but emails can be sent to either and be forwarded to the mailboxes on domain-two.co.uk
There is no built-in feature that allows you to configure a forwarder on a per-domain level that will allow you to forward emails from the corresponding mailboxes, i.e from [email protected] to [email protected]. There's an open request you can vote for here. As a workaround, the forwards need to be configured per mail account: