carrying backpack(s) on the rear rack

Anything to be concerned about?

We plan to ride and hike. There would be 3 possibilities:

1. Carry frameless backpacks in out panniers. Wasted space and not great on the trail 2. Use the Ortlieb's carrying system (https://www.rei.com/product/798959/ortlieb-panniers-carrying-system). Same...