I have found that in all my years of use with Macs, multiple OS versions, multiple hardware configurations, that it never queries anything but the primary DNS server. Why is this? We have spun up many different internal DNS servers and have tried specifying both internal and external DNS servers on the client. It will only resolve hosts in the primary DNS and completely ignores any DNS specified after that.
If it gets a response from the first DNS lookup it won't try again with another DNS server, even if the first response is NXDOMAIN. All your DNS servers are supposed to return the same results; if you're relying on different DNS servers with different results then your architecture is broken.