I figured out a solution after reading the last post on this page. For some reason I didn't think about about it, but I can just mute what ever device I'm not using at the time when using the splitter. Then I wouldn't have to worry about the headphones outputting audio when I'm using the speakers.
I just got some ATH-M40x headphones to replace my old Siberia V2 headset. With these new headphones I can hear the static MUCH more loudly in my front panel jack than with the old ones. My on-board sound card of course only has one line out jack, and I want to be able to have both the speakers...