Converting H.264 files with 12 audio channels to HEVC without mixing down to one stereo track

I'm sorry if the title is a bit run-on. My brain is tired from trying to do this.

So I have a LOT of very high bitrate footage in H.264 that has 12 audio channels. 7TB later I've learned my lesson but now I have TBs of bloated media. I've found that converting the footage to HEVC in AME reduces the file size blah blah blah you know this part. 

The problem I am having, and hoping that you can help with, is with keeping the 12 audio channels intact during the export. I want to get rid of the bloated original media once I've converted everything because my projects just aren't sustainable at the rate I'm gathering media. 

I know I could export it twice to work around it, but I'm hoping I'm missing something so everything can stay synced.

EDIT: 2 days later and 7 people have seen this post. How is this forum this dead? Guess I should stick to reddit. For those who are curious as to what I did to resolve my issue, I just cued up a high bitrate HEVC and a 12 channel WAV preset for each of the media files I wanted to reencode. Worked like a charm, took 2 days of my computer's full attention, but I ultimately freed up almost 5TB. If there's a way to select more audio tracks in HEVC in AME, please let me know.