RFC 2182 details the operation and functionality of secondary DNS servers. Which defines/influences how resolvers behave with multiple DNS servers.

When you specify more than one DNS server for network clients, the order in which these servers are used is not random. The following sequence is observed:

Primary DNS Server: Clients will first attempt to use the primary DNS server specified in their network settings. This server is often considered the default.

Secondary DNS Server: If the primary DNS server is unavailable or unable to resolve a query, clients will then attempt to use the secondary DNS server. This server acts as a backup.

So in OPs case his Pihole will always be used if it’s available, and only when it’s not will the secondary be used.

You’re lucky, I’m on 25/2Mbps and I pay £32 per month for the privilege.