ok. this may sound arty farty, but just follow me for a second here.
so basically with a 3 band eq you can control the tunes in 3 dimensions.
The low band is the power of tune, the mid is the presence, and high the "sheen".
With this in mind, you can either keep the balance between two songs, or create your own breaks, drops and tension moments.
As said earlier, you dont want two tunes playing their basslines over one another. Like wise, you dont want two busy tops with cymbals and crashes flying all over the place. Make room for the tune you want to stand out more by cutting the same frequencies on the other. Think of it as layering two pictures in photoshop: one will be the background, which will fill the picture, and the other will be you focal point, which it might not fill the whole view, but its what stands out from this composition. How you do that ? you make the background more transparent.
another advice is if your mixer has kill switches, NEVER use them. these bloody annoying things are found only in the cheapest of mixers,or older ones, like the vestax pcv 270. in any case, they dont give you as much control as you need. Its all about lowering frequencies, not removing them completely. Also, after a bit of use, they tend to fail, and produce a very annoying click every time you throw them. They just suck. Get used of turning the eq knobs with a swift move, and soon they'll become a switch on their own.
another favourite use of eq for me. Wait till the last bar of a phrase, and whilst you have the foreground tune playing with neutral eq, and the background tune, with bass cut off, and tops a bit lowered, turn the foreground tune's fader off, and lower the mids whilst increasing the tops of the other tune, and at the beginning of the next phrase, throw the other tune back in. What youve just done is a resonant high pass filter sweep, without a filter. everyone in the club is screaming your name, guys are bigging you up, wannabe dj's are gobsmacked and fuming with jealousy, and gilrs want to have your babies.
= success.
dont forget to mention ol' gord once your famous.