I have the main output through the sound card, but I can't get the preview on the MX2 at the same time. I'd like to get the master output through the interface and the preview through the MX2.
Traktor only supports one audio interface at a time. Either the internal one of MX2 or your external one. If want to use MX2 headphone sockets you‘ll have to connect your speakers to your MX2 too.
On a Mac you can create aggregate devices which consist of more interfaces but appear as one (virtual) audio interface to apps like Traktor. But this doesn‘t always work flawless and can lead to issues.
Not sure if there is a similar option on Windows too. Or a third party tool to make this possible.
Asio4All can aggregate audio devices in Windows
You can use this tool on Windows:
https://vb-audio.com/Matrix/
