Do you need that clients receive two default gateways instead of one?
Could you try to set them like this?
db dhcp setprop enp0s3 DhcpGatewayIP 1.2.3.4,5.6.7.8Substitute enp0s3 with your interface (you could press TAB to auto-complete) and the two comma-separated ip addresses.
Then
service dnsmasq restartPlease, let me know, as we could relax the interface validation.
UPDATE: I tested it and it works as expected.
