Reading the upgrade documentation / recommendations it states that you should always update your target filer first when snapmirror is in place before upgrading your source filer.
What is the correct procedure when you have both filers snap mirroring data back to each other? We have data at our d/r site the sends itself back to the corporate site via snapmirror and we have data from corporate going to the d/r site via snapmirror.
Could someone give some insight as to the best way to approach this?
There is no easy answer. Depending on the amount of time between the two system upgrads to 8.x you will need to disable the schedules on the specific bi-directional SnapMirror relationships or disable SnapMirror on the systems.