The legend rule shouldn't apply until the second Mindslaver has resolved.
The Legend Rule said:420.5e If two or more permanents with the same name have the supertype legendary, all are put into their owners' graveyards. This is called the "legend rule." If only one of those permanents is legendary, this rule doesn't apply.
The second Mindslaver would have been a spell, and not a permanent, until it resolved. At that point, if there are two legendary permanents in play with the name Mindslaver, they both go to their owners' graveyards as a state based effect.