Unable to add more than 1 static route
If you want to set additional routes either add all routes:

networksetup -setadditionalroutes "Ethernet 1" 10.0.0.0 255.0.0.0 69.69.69.69 20.0.0.0 255.0.0.0 69.69.69.70


or set a variable with the existing additional routes:

ADDITIONALROUTES=$(networksetup -getadditionalroutes "Ethernet 1")


which yields e.g.

echo $ADDITIONALROUTES
10.0.0.0 255.0.0.0 69.69.69.69


and then use 

networksetup -setadditionalroutes "Ethernet 1" $ADDITIONALROUTES 20.0.0.0 255.0.0.0 69.69.69.70


to add other routes without loosing the old routes.