Mix/DJ with just 1 CDJ 2000 Nexus

Sure - you could use an application such as Traktor or VDJ and map the controls such that one of the buttons (eject, perhaps?) is the MIDI signal for "Toggle Focus" and thus the play button would work for each deck. The only problem you may have, depending on your setup configuration, is how the audio is output from the computer. If you're using a DJM with a built-in audio interface, then you would send each channel from the software to a different channel on the mixer and that's no problem.

i recommend you to buy a completly set. My opinion is that is a bad solution and to expensive. You can play with different ways like the CD Player, Controller, Vinyl-Timecode and you must decide for youself, what is the best way.

One of the CDJ 2000 Nexus cost 1.900 € and a controler like the DDJ-SZ cost 1.000 € it is the cheaper way and the better one. This is only an example. 

Your way is posible but not recommendable.