Here's my stab using a parser.

#!/usr/local/bin/perl

use strict;
use warnings;
use HTML::TokeParser::Simple;

my $data = do{local $/; <DATA>};

my $p = HTML::TokeParser::Simple->new(\$data);

my ($in_tag, $in_g, @AoH, $tag, $txt);
while (my $t = $p->get_token){
  $in_tag++, next if $t->is_start_tag;
  $txt = $t->as_is if $t->is_text;
  if ($t->is_end_tag){
    push @AoH, {
      tag => $t->get_tag,
      txt => $txt,
    };
    $in_tag--;
  }
}

while (@AoH){
  my @rec = splice @AoH, 0, 2;
  print '<au>';
  printf "<%s>%s</%s>", $rec[0]->{tag}, $rec[0]->{txt}, $rec[0]->{tag}
+;
  print ', ';
  printf "<%s>%s</%s>", $rec[1]->{tag}, $rec[1]->{txt}, $rec[1]->{tag}
+;
  print '</au>,', "\n";
}

__DATA__
<surname>Coff</surname>, <givenname>D.,</givenname>, <surname>Coff</su
+rname>, <givenname>D.,</givenname>
<givenname>D.,</givenname>, <surname>Coff</surname>, <givenname>D.,</g
+ivenname>, <surname>Coff</surname>
[download]

outputs:

<au><surname>Coff</surname>, <givenname>D.,</givenname></au>,
<au><surname>Coff</surname>, <givenname>D.,</givenname></au>,
<au><givenname>D.,</givenname>, <surname>Coff</surname></au>,
<au><givenname>D.,</givenname>, <surname>Coff</surname></au>,
[download]