You might be able to load two players, set each one on different Left or Right channels. Have the output thats going to the house speakers on say the right, and the headphones on the left. You might need a " Y " adapter so you can plug both speaker and headphone lines into the same jack.
Another option you might want to install 2 soundcards for this.
Here is a FREE program that seems to be working well for such purposes, the only draw back is that it may not support the wma format, but if the app works for you in testing you may email the author and suggest he look into the WMA support, i have already done some bug testing for him on the ogg format.
The other thing about this app that makes it ideal for DJing versus say Winamp, is the database, which allows you to find songs faster. here are a quick list of some option it can perform[*]prelisten function for headphones (needs second soundcard)[*]realtime-mixing of two mp3-Files[*]saving Cue-Point for every song (Autocue)
- Nicks DJ Workstation
http://dj-workstation.de/English/start.html