Perhaps I'm missing something here. If you don't want both of the Listboxes to be scrolled, then don't create them both as scrolled Listboxes.

use strict;
use Tk;
use Tk::TiedListbox;

my $top = MainWindow->new;
my $f = $top->Frame->pack(-expand => 'yes', -fill => 'both');

###########################################################
#    2 scrolled listboxes
my $l1 = $f->ScrlListbox(-relief => 'flat',
                         -exportselection => 0,
                         -selectmode => 'extended',-height => 20);
my $l2 = $f->Listbox(-relief => 'flat', # <-- Not Scrolled!!
                         -exportselection => 0,
                         -selectmode => 'extended',-height => 20,
                         );

$l1->pack(-expand => 'yes', -side => 'left');
$l2->pack(-expand => 'yes', -side => 'left');

# tie the scrollbars and selection
$l2->tie([$l1->Subwidget('scrolled')]);


##########################################################
# 
my $id;
foreach $id (0..300){
  $l1->insert('end', $id);
  $l2->insert('end', $id);
}

MainLoop();
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