OK so one option is to go with this and this.

The splitter amp would permanently feed the AV IN for the front screen, and the 2:1 switch. The other outputs are unused.

The 2:1 switch would be fed with AV OUT from the GPS.

Hence front seat passenger can select AV IN to see the DVD.

Rear seat passengers would toggle the 2:1 switch to see the nav or DVD.

I couldn't find a 2:1 switch with a neat looking remote button, but I'm sure they exist.

If there is a space video audio out on the thing that feeds both monitors then you won't need to use the amplified splitter.