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.