I'm looking for a USB 2.0 audio interface that allows simultaneous multiple stream format playback. For example if I have the ASIO driver running with my amp sim software I want to be able to play along with and hear a Guitar Pro track at the same time, for example, without either of them cutting away. Does such audio interface exist?

I know the E-MU's don't work like that because it specifically mentions it in the manual for the 0202 and the 0404. "The 0202/0404 USB 2.0 only supports playing back one stream format at a time on the PC. Each of the three mentioned stream types has an associated priority. If a higher priority stream type is opened while a lower priority stream is already playing, the lower priority stream will stop playing. The stream priorities, from highest to lowest, are ASIO, WDM"
i want to say the lexicon interfaces allow such functionality(i worked with the alpha). what you generally need to do is 'release the audio driver for background applications' or something, from what i remember, their interfaces start for around 100 usd. might want to do backup on that info though.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
Currently, I have an M-Audio PCI sound card. Whenever I load the M-Audio ASIO driver in my DAW (FL Studio) and choose the Auto Close Device option (which I imagine is just like releasing the ASIO driver to background) it doesn't do what I want it to do. Sure it lets you switch back and forth between windows, but you can't work with things simultaneously.

After digging a little bit deeper I guess the name for what I'm looking for is called "multi-client support" and so far the only interface I've found that seems to be able to do what I want is the Native Instruments Audio Kontrol 1. That can't be the only one...can it?