Cubase - Devices/Device Setup/Your soundcard/tick release driver when application is in background.
BUT...this will allow you to play whichever is on top. You cannot play both at the same time because only one application can access the soundcard driver.
offki's advice may let you do this if you really want to but IMO it's hard enough getting a soundcard driver running efficiently with a DAW as it is, so sharing it isn't something I would recommend for everyday use.
The instructions were basically correct for Win7, I just checked....but to spell the process out exactly:
Ctrl Panel/Sound/right click or mouse select your soundcard/properties/advanced/untick the exclusive mode boxes.