Sure, you can. Dnsmasq has an option 'strict-order' which instructs dnsmasq to always forward requests to upstream servers in the order you specified. In your case, put 192.168.x.x first and google the second. Use this CLI to set the dnsmasq option:
set service dns forwarding options strict-order
Alternatively, your pihole or dedicated dns server/forwarder shall have redundancy already. Hence, I won't run dnsmasq on edgerouter. Specify 192.168.x.x as system name-server on your edgerouter instead. Setup DHCP on edgerouter to tell clients to use 192.168.x.x as dns server. That's how I did my LAN at home.
