601.7 Some spells specify that one of their controller’s opponents does something the controller would normally do while it’s being cast, such as choose a mode or choose targets. In these cases, the opponent does so when the spell’s controller normally would do so.
601.7a
If there is more than one opponent who could make such a choice, the spell’s controller decides which of those opponents will make the choice.
601.7b
If the spell instructs its controller and another player to do something at the same time as the spell is being cast, the spell’s controller goes first, then the other player. This is an exception to rule 101.4.
(refs: 101.4)