There would not be a need for another shortcut. Simply a designation that gaps can be edited into secondary storylines as well as primaries.
For example, if you want to insert a clip from your event browser into your project, you hit W. If you want to do the same into a secondary storyline, you simply select the storyline bar to make it active and then hit W. It's the same shortcut for both functions, you're just telling X which storyline you want to insert the clip. That's what I'm talking about.
Andy
http://www.timesavertutorials.com