Similar to many ... but my old formula for multi-day events my 2nd GPS would contain my full planned route so I had a decent picture of actual arrival ETA while my primary would be for point to point, so I could easily tell if I had time to add in or had to remove something. All backed up by handwriten index cards in my tankbag window of course.
I finally added in a phone and 1st used it in LDX, so that became my point to point, using POI Viewer and my secondary GPS held my actual route. For me, this worked great, I found routing on the fly was easier with my phone than the GPS so that is the system I use now, and how I was set up for the last IBR. I have the BMW Nav for GPS so I usually have that on the bikes telemetry data screen until I'm in a city.
As far as redundancy, for long events, I carry a spare Nav with my routes already loaded. It only takes a few minutes more to load into 2 units so if mine bricks up or wonks out, I can simply swap it out roadside and move on.