{
   package MyYViewMultiplexer;

   sub new {
      my $class = shift(@_);
      return bless([ @_ ], $class);
   }

   sub yview {
      my $self = shift(@_);
      $_->yview(@_) for @$self;
   }
}

$scrollbar->configure(-command => [
   'yview' => MyYViewMultiplexer->new($listboxA, $listboxB)
]);