Can I use both Victini's "Victory Star" Ability and Glimwood Tangle stadium to reflip coins from an attack twice?
No, you cannot. You can use either the ability or the stadium, but not both of them. As a general rule, you can't use more than one reflip effect at a time.
Source: TPCi Rules Team (2020-09-24)

Relevant cards

Note: All applicable cards may not be listed below.
BW3_14
BW3_98
SWSH3_162
SWSH3_162