You have drawn a conclusion about what those checkboxes mean. Both Google and Cloudflare have more than one DNS. Google's are 18.104.22.168. and 22.214.171.124 while Cloudflare is 126.96.36.199 and 188.8.131.52. The checkboxes just indicate which of those you want. Both of each should be roughly equivalent, performance-wise.
Selecting more than one upstream doesn't really give much benefit. Which ever one is 'better/faster' as determined by the underlying dnsmasq that pi-hole is based on, will be used (I think).
Improve detection algorithm for determining the "best" forward destination¶
The DNS forward destination determination algorithm in FTLDNS's is modified to be much less restrictive than the original algorithm in dnsmasq. We keep using the fastest responding server now for 1000 queries or 10 minutes (whatever happens earlier) instead of 50 queries or 10 seconds (default values in dnsmasq). We keep the exceptions, i.e., we try all possible forward destinations if SERVFAIL or REFUSED is received or if a timeout occurs. Overall, this change has proven to greatly reduce the number of actually performed queries in typical Pi-hole environments. It may even be understood as being preferential in terms of privacy (as we send queries much less often to all servers). This has been implemented in commit d1c163e.