Okay I have played around with this even more and its just not going to work without fixing isc-dhcp itself. Here is an example configuration, where the gateway is set in one shared-network and not in the other. When the machine boots it gets the gateway from the other shared-network.
Now if you re-order the configuration and place the one without the gateway first it works as expected. But this will not work as the ordering can never correct as described in my previous comment.
Okay I have played around with this even more and its just not going to work without fixing isc-dhcp itself. Here is an example configuration, where the gateway is set in one shared-network and not in the other. When the machine boots it gets the gateway from the other shared-network.
http:// paste.ubuntu. com/15273844/
Now if you re-order the configuration and place the one without the gateway first it works as expected. But this will not work as the ordering can never correct as described in my previous comment.