Yes, if you have ASIO drivers, you can do something like aggregate sound cards I think. Can’t help you with the specifics as I haven’t done this in forever.
Since the UM2 is a 2+2 card (two inputs for mics/instruments and two outputs = ONE stereo!), it can never be used in a normal DJ manner, which calls for at least 4-channel = TWO stereo output.
While the UM2 has a headphone connection, that is used to monitor your incoming/recording sound or it will parallel the output for track playback from a daw. In most cases this headphone channel can not be addressed directly from DJ software.
With the price of a good DJ soundcard being relatively high, I’d always advise someone that doesn’t have a controller yet (or one without a soundcard) to get a controller with a soundcard and be done with it. Pricewise it shouldn’t be too much of a difference. Clearly we are not talking about 2+2 Behringer or other low end brand 55 euro sound cards here 🙂