If I have Focus Sash attached to Marshadow & Machamp GX and then use its "Acme of Heroism GX" attack, which one takes precedence if Marshadow & Machamp GX would be KO'd by damage from an opponent's attack?
You would choose, as they have the same effect, you choose the order of resolution.
Source: TPCi Rules Team (2019-06-27)

Relevant cards

Note: All applicable cards may not be listed below.
SM10_198
SM10_198
SM10_199
SM10_199
SM10_221
SM10_221
SM10_82
SM10_82
XY3_91