Hello,
The M800 is perfectly capable of running DBW directly, there is no need for a DHB. Aux 1 and 2 are specifically used for full bridge control of a servo motor, mainly DBW. I have done many ETA2 set ups and this is what I would recommend.
The main thing to remember is that your spring load when fully open will set your full throttle (continuous current) which should not exceed 2A.
Please watch my Youtube video on this subject as there are a few things to take into consideration.
https://www.youtube.com/watch?v=ID7eu-F1eqA&t=2s