I would create a workflow with the following logic and have this trigger on item creation and when item is modified.
Add a column to the list called "Due Date Reminder Email"
If Due Date Reminder equals Today,
Send Email to (Relevant person)
Then update Due Date Reminder Email to value "sent"
If Due Date Reminder Does not equal Today
Update Due Date Reminder Email to value "Pending"
(Logic here is that the update will trigger the workflow again due to being set to trigger when item is modified)
If Target Date equals Today
and if Completion Date Not equals Today,
Send Email to
This may need tweaking slightly but this is going off my assumptions for the way you have your columns set up. This will also send the reminder daily rather than every other day. Hopefully this helps or gives you a decent starting point at least.