how do i get a stereo signal from minibrute`s mono audio output?
You don't. The MiniBrute provides a mono signal (which is routed to both speakers when using the headphone output). It is not a stereo instrument.
If you want to use both inserts of your soundcard you might use an adapter with a stereo 1/4" jack (plugged into the headphone socket) on the one side and 2 XLR plugs on the other one. However, this will give you 'double mono' (i.e. the same signal on both channels), but not stereo.
However, I suggest using just one of the XLR sockets your soundcard provides and leave the rest to your mixer
Have a look here
as well, 2nd reply (by Bruno@arturia).