Here is a fun accessory for your iPhone or iPod, the iPhone DJ Mixer, which lets you mix music from two iPhones or iPods or Mp3 Players.
Mix things up with this home mixer that lets you combine music from 2 iPods or mp3 players, or one of each. AC adapter; spin option; 2 effects programs with reverb, flange and filter and cross-fade slider; recording option and speaker system connection with a stereo audio cable with in-line jack;. Charges your iPod while it plays; compatible with a microphone.
If you want one the iPhone DJ Mixer is available for $120 from Urban Outfitters.