There are multiple hardware combinations which can be achieved with Serato DJ Pro, so its worth pointing out what is supported and what isn't:

There are now various available for use in Serato DJ Pro such as the CDJ2000 nexus that allow platter control via a USB connection. These devices were intended for use with DVS hardware only. It is possible to get them to work with controllers although this is not formally supported and has not been tested completely. There are some strange interactions that can take place when attempting to control the same decks on both pieces of hardware. Use these configurations at your own risk in performance situations.

We still support the use of 3rd party MIDI controllers alongside supported audio and MIDI hardware, ie. using an Akai LPD8 with a Pioneer DJM900 SRT as an audio device OR a Pioneer DDJ-SX as an audio/MIDI device. However, we do not support the use of 2 native devices at once, ie, a Pioneer DJM900 SRT as an audio device AND a Pioneer DDJ-SX as a MIDI device. Once again this is because the results are unpredictable and you may experience strange interactions.

At any stage if you do connect 2 native soundcards or MIDI devices at once the piece of hardware that successfully connects first will be used by the software as the Audio device. The second device MAY have some MIDI functions still available for use but these hardware configurations are not supported so the results are unpredictable and should only be used at your own risk in performance situations.