Menu: Tools -> Options -> Environment -> Documents
Normally, when you open up a new document it creates a new tab:
But if you go to Tools -> Options -> Environment -> Documents and check the "Reuse current document window, if saved" option then you get the following:
The caveat here is you have to have a "clean" (saved) document for this to work. If the document was "dirty" (not saved) this would be the result even with this option turned on.
Before (note the asterisk to indicate the document is dirty):