my @a;
my @b;
GetOptions (
  "a=s" => \@a,
  "b=s" => \@b
);
@a = split(/,/,join(',',@a));
@b = split(/,/,join(',',@b));

if ( scalar(@a) !=2 || scalar(@b) !=2 ){
  # shout at user here
}