Would something like the following work?

{
   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)
]);
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Of course, that's a hack. The normal way is to have one list with two fields.