Isn't this as simple as running the script every Wed and Fri (say as a cron job), and sending an email for every due date that's [0..14] (or [1..14]*) days away from today?

For the last reminder,
On Wednesdays, check if the due date is [1..2] (or [0..1]**) days away from today.
On Fridays, check if the due date is [1..5] (or [0..4]**) days away from today.

* — If you don't want notifications on the due date.
** — If you want the special notification be to sent before the due date.

Update: Untested code:

use Date::Calc qw( Day_of_Week Delta_Days Today );
use List::Util qw( min );

my ($ty,$tm,$td) = Today();

for (...{ tasks }...) {
   my ($dy,$dm,$dd) = ...{ due date of task }...;

   my $delta = Delta_Days($ty,$tm,$td, $dy,$dm,$dd);
   next if $delta < 0;   # Don't care about past-due tasks.
   next if $delta > 14;  # Don't care about due dates that far ahead.

   my $ddow = Day_of_Week($dy,$dm,$dd);

   # $cc_super is true if this is the last
   # Wednesday or Friday before the due date.
   my $days_to_wed = (3 - $ddow) % 7;
   my $days_to_fri = (5 - $ddow) % 7;
   my $days_to_next = min $days_to_wed, $days_to_fri;
   my $cc_super = ( $delta > 0 && $delta <= $days_to_next );

   send_reminder($cc_super, ...);
}
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Update: Fixed typo in code resulting in the error mentioned in reply.