For me:

WTG or going downward
=> right hand / right side of face, left hand / left side of face

ATG or going upward
=> right hand / left side of face, left hand / right side of face

You can experiment with other variations of course - whatever works for you! Changing hands can allow better visibility and easier access. (Of course, you can still do it all with one hand only).

If you choose to use both hands at the start, you'll find you get used to it pretty quickly - both hands are getting used to the whole experience.

I'm finding some things easier with my 'off' hand - it just takes practice and experimenting.

All the best,
Michael.