DJ Dan Shaughnessy asked for advice on switching from CDs to MP3s.
If you're DJing off of CDs or phonographs, you need two players so you can cue one up while the other is playing, as well as for tricks like crossfading (fading out one song while you fade in another). But once you've got the music on your hard drive, no such duplication is necessary.
Indeed, you can even do limited DJing in Windows Media Player, although anyone serious about the art (full disclosure: I don't qualify) would scoff at the idea. In WMP 11, click the Now Playing tab, then the tiny arrow below "Now Playing." Select Enhancements, then Show Enhancements. This will bring up a small control panel at the bottom of the window. The other options on the Enhancements menu let you select this control panel's functions, including crossfading and speed control.
Of course, there are real DJ programs for those who want or need them. Although I don't really have enough expertise to give a serious recommendation, I'm going to suggest you check out the free Zulu DJ. While it has some annoying shortcomings (adding songs to its library is more of a hassle than it should be), it can do all sorts of effects and is generally easy to figure out.