I don't use 2 GPS units on the bike, but I do on my trips up and down the east coast in the car.

Why?

One shows the map, the other displays speed, distance covered, moving/stopped time, avg moving speed, overall avg speed.

the one doing the navigating will show distance to destination and arrival time and the posted speed limit

Come across 301 in Florida and you are going to use the GPS speed limit display to keep you from getting nailed for going one MPH over the speed limit.

It is not much different than a small plane where you use multiple navigational units and displays. I can think of 4 in the PA-28 I fly right off the top of my head

 

Okay to answer the most popular question as to why...I like to ride in the scavenger hunt rallies,(Heart of Texas, Big Tex) etc. and I use the 665 as my primary routing device, and I use the second 660, to search for bonus locations. Allot of planning goes into these rides and routing to be efficient with your time is important. The second device allows you to search for other locations in case you are ahead or behind schedule. More time ridding and less sitting looking at a map.

 

Dang. I was going to guess that if you have two of those and you look at them thru special glasses, you get an awsome 3D experience.