Xtrabackup itself doesn’t need to talk to each other… just is used to backup/restore 5.5, then you can upgrade in place on the new servers and set up replication. Replication from 5.5 to 5.7 that’s not supported but it normally works fine most of the time (there might be edge cases so you need to test). Worst case you can set up an intermediate 5.6 server.