With the v4 rewrite - it appears it may be easier to support multiple peers to keep 3 or more pi-holes in sync rather than only two. Prior to v4 this was easily accomplished given two+ secondaries could connect to a single master pihole. However, given the re-write to peering and use of non-unique md5 hash file names it appears this usecase is no longer supported.

This can likely be fixed by ensuring the md5 hash files are more unique in addition to allowing multiple hosts to be configured.