Scott,
Glad you're getting close! If I understand you correctly, you need to also filter your list by contacts that have been invoiced in the last two years. This is a little more complicated, but definitely doable!
First thing you will need is a new field in your Entity table. It could be a Yes/No field called RecentlyOrdered. The trick is to loop through the Invoice table and update your RecentlyOrdered field to Yes where the DueDate is greater than two years ago. To make things a little simpler, you could put a DatePicker object on your screen, and loop where the DueDate is greater than the value in your (Value From Screen) DatePicker object.
Make sure you link your RecentlyOrdered field from your Entity table into your Contacts table, and then you can filter your Contacts report by that field!
Try this, but let me know if and where you get stuck and I can explain a little further.
Jason