Help Creating Output Aggregate Device

I'm not sure about this, but just in case nobody more knowledgeable chimes in:

I think it depends on the machine. I know that the MacBooks (and I assume other recent machines) can output to both, because in some cases they did under Windows (Boot Camp), which isn't usually the desired effect. Older Macs, however, I THINK may have used a hardware "switch" to override the built-in speakers if something else was plugged in.

I remember that under some versions of OSX if you plugged in a pair of headphones early in the boot process the Mac would panic, but that's neither here nor there... just speaks to there being a hardware signal that the Mac read to determine what was plugged in where.