im pretty sure master tempo on the cdj 2000 does that. so normally if you move the pitch fader up and down it will affect both tempo and key. now say you have song 1 at 128 bpm and song 2 at 128 bpm, but song 2 is in a key that does not match song 1's key. so, you raise the pitch to a key that matches song 1, however unfortunately song 2 is now at 131 bpm due to increasing the pitch. Now you hit master tempo and the cdj will keep the key of song 2 when its at 131bpm but now you can adjust the bpm back down to 128bpm without affecting the key you thought sounded best with song 1. its alot easier to show this then to explain so here is a video i found. if you skip more towards the end it shows exactly what i am explaining.