SQl Developer Single Query result tab please
So, it turns out this is a bug. If you look closely, you'll notice that for each query run, your results tab is automatically being pinned, causing each new query run to need it's own new tab to display the results.
The automatic pinning is only supposed to be enabled when you explicitly go into Tools->Preferences->Database->Worksheet->"Automatically Freeze Result Tabs".
It is an identified bug, that Oracle fixed with SQL Developer 2.1.1.
Go to oracle.com and download the latest version and this should go away. It was definitely very annoying.
SQL Developer 4.0 (2013)
Close all the Query Result tabs
Tools > Preferences
Database > Worksheet > uncheck "Show query results in new tabs"
Instead of "Run Statement" click on "Run Script"(F5). This would give the output of two queries in the same Query Result window.