@Maria369,
the new booking request from the 2nd guest blocks your calendar, so if you want to send out another pre-approval for the first guest you previously declined then you first need to decline the 2nd guests booking request. After this you calendar will be free again and you should be able to send out your pre-confirmation.
Your first guest needs to be fast with accepting your pre-approval, otherewise the same situation could happen again when a new definive booking request will come in...