Okay this came up today. The rule book says the person who's turn it is determines the order of simultaneous effects. If I target say a character with Mastermind and a another character can I the attacker decide who gets clicked first? For instance if a figure is on their nlast click can the attacker decide to click them first so the defended can't use mastermind?

Nope, there's a specific rule in the PG to deal with damage dealt.

"All damage from a single effect is dealt simultaneously, unless otherwise specified. For example if two characters are both hit by the same Energy Explosion or Throw a Grenade attack, all of the damage from that attack is dealt at the same time, so any damage transfer will be combined."

The order of clicking doesn't much matter as it doesn't happen until all of the damage dealt is distributed.

Damage is dealt at the same time.

So if you do 3 damage to each character and one character mastermind's it to the other. The second character just takes 6 damage.