Looked around, apparently it's not possible. TF2 probably uses different channels for voice and normal audio but any program that would isolate those would probably be detected as a cheat program by VAC.
 
Lol, Linux through Wine can actually do this sorta shit. You can listen to TF2 on your headphones or TV, and micspam through your TV or headset.

But only if it's Digital/Analog, because I think it's impossible to separate 2 different sounds through the same output device.
You can do it in windows too, just not natively. Yes you can.