Mix Down Question

How do I mix multiple AUDIO tracks into a stereo AUDIO track before adding mastering plug-ins the exporting?

I can do the Mix Down via the Export process (MP3) and the mixdown via the Group Function but I want to be able to mixdown (in the box) into a stereo audio track so I can adjust gain on a track or 2 while mixing real time into stereo.

I have done this in the past but I have memory issues beyond what my 57 years would normally provide.