So say that you are arguing about how much something changed over the years and you split your documents into two different groups and then use these two groups to argue that the thing has significantly changed over the years. For your outside evidence, can you make it another group (a separate paragraph) that you use to briefly support your argument about how the change was significant? The outside evidence wouldn’t be related to the other two groups, but it would work to support the main argument and it would be part of your thesis.
The patterns of trade significantly changed from the period 1200 to 1750 because countries switched from using paper money when travelling through trade routes to silver based trade and patterns of trade changed because humans began to be traded in addition to luxury goods. The patterns of trade also significantly changed because technology that was used for trade was innovated on and changed over the years.
The last supporting claim about technology changing would be solely supported using outside evidence instead of documents.