Here's an answer (if you're still looking):
sudo nano /etc/dnsmasq.d/02-pihole-dhcp.conf
dhcp-option=6,Pi-holeIP,SecondaryDNSIP
sudo service pihole-FTL restart
You would have to renew release on each host after that to pull the new settings.
Consider running tests with namebench with various combination of DNS servers. Set the fastest one in pi-Hole and a slightly slower one in your secondary DNS resolver (i'm assuming it's your router). I am only talking milliseconds here based on the namebench results.