while examining the debugging capabilities of SQL Developer I stumbled over the option to specify a Break Point Option [Break Only for Threads Named] folowed by am empty select list.
It is selectable when hovering with the mouse over the break point icon (left of the code area) and then clicking on the more button.
How can I specify a thread name in PL/SQL?
Don't believe you can - it will default to 'main' - you can see this in the Debugger Call Stack panel.
This MIGHT be more relevant for you if you're doing external debugging.