Is it possible to send the task reminder (due date) email prior to sending out the initial due date email?
Yes, you can set a reminder notification to be sent out a specified number of days before a date entered in a date field. You have to have a variable to store the date and add a Calculate Date action and that is where you will tell it the number of days prior. It would look something like this:
You would then have a run if that would contain the email notification if the date matches your variable.
I hope this helps. Let me know if you need more information.
sticking to task notification options you are unable to send different reminder emails and then different "due date" notification email... You can only send the initial email and then reminders. It you set due date, and your task won't get completed before then you can as well prepare the message informing user his action is not needed anymore, but that would result as well in automatic completion of the task thus the assignee won't be able to interact with the task.