Is Rillaboom VMAX's "G-Max Drum Solo" or Cloyster's "Aqua Split" attacks' bench damage amount split between the two chosen Pokemon (20+20 or 10+30, etc.) or does it do 40 or 30 damage to each of them?
The attack does 40 or 30 damage to each of the two chosen Pokemon.
Source: Fusion Strike FAQ; TPCi Rules Team (2021-11-11)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH8_51
SWSH8_51
SWSH8_23
SWSH8_23