No, you just plug the turntables into the black box, and then plug the black box into your pc's input on the soundcard. Then you go out of the output of your soundcard to your amplifier, then to your speakers. Or if you have active monitors, out your soundcard into the amplifier on the back of your active speakers. you don't need dual soundcards. The Rane TTM56 is a fantastic mixer and would still be awesome mixing house music, trance or anything. Its only if you want to use filters to manipulate the sound (like house djs do all the time) while you're dj'ing, that you'd need a different mixer. Personally, I never manipulate filters on dj mixers. I just leave the song playing, because if you use filters all the time, it ruins the song people are trying to listen to.
But, if you want to filter the songs live, then try and Allen & Heath Xone 42 pictured here :
Sorry for the large image, but it gives you a brilliant view of this beautiful mixer.
And available here :
http://www.htfr.com/more-info/MR315311
Or if you're from the usa, here :
http://www.uniquesquared.com/servlet/the-680/Allen-and-&-Heath/Detail
Its a fantastic mixer, has filters, excellent sound quality (near Rane quality, but not quite a s good), and has a 'very sharp' setting for the crossfade for scratching. Its a bit more mkoney, if you wanna scratch and also mixe house, trance, hard house, uk hardcore, anything, this maybe the mixer for you. Plus, as a bonus, its got excellent channel input metering, which means you can definately keep an eye on the level at every stage of the mixing process.