If you mean on PC, then a player like Videolan will let you open 2 instances of the player simultaneously, allowing you to hear both simultaneously (just make sure the option for 'Allow only one instance' is NOT selected):
Videolan also allows 'drag-&-drop' creation of simple playlists, on-the-fly.
thank you. on a laptop there are many choices and you pointed out a good one. but i am iterated in doing this w my iPhone. I tried edging mix, DJplayer pro, and cross DJ. However, these apps are designed for short mp3s. And I am having trouble adding a bunch of songs to a queue so that 40 classical tracks play while my 500 meg audio book plays in the other slot. Their UIs are pretty complex. I don't need all the fancy effects. Does anyone know which is the simplest DJ app?