how does the heal mastery work with the double charm spot?
and does it show up as extra heal dmg?

The only stat that works in the second charm slot is enhanced effect. If you use a strong heal mastery charm in your other slot, it is possible to get nearly the same heal power as if the charm were in the main slot. The main benefit of this is that the charm in the ACC slot now uses only 75% mana cost, in addition to being only 75% effectiveness.

ok, ty for this clear anwser!

just one more thing:
does the increased healing show up in the charm?

No, it'll only show up on your actual healing actions. You'll have to test it out to see what kind of results you get.

vouch