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Hi Micha, 

thank you for the question. 

If I understood correctly, you have question about invoice closure. 
When you’re using the invoice tracking function in Mews, you have indeed two documents after the invoice is paid. One is an invoice, which is sent to a customer as a request of payment, and the second one is a bill, where the payment is collected and closed. 
The invoice itself has the tax date of when you issue the invoice. The bill however, has a payment on it that was collected (and consumed) in a later date, and the bill itself is as well issued only after the payment is received. Because of that, the bill reflects that date and it cannot be issued with retrospective date.

As these are two separate documents, they are indeed issued on separate dates. 

That being said, you do not need to use the invoice tracking function within Mews, as long as you keep track of your invoice payments elsewhere (such as in an accounting software, etc). In such scenario, there would be only one document issued in Mews, and that is the invoice itself. As the payment would not be tracked in Mews, there would be no payment input in the system, and therefore no second document. You may see more details in this article. 
 

I hope it helps :)