The SYNC variable in /etc/make.conf should be able to take alternative values (maybe through a SYNC_2, SYNC_3,... or SYNC_ALTERNATIVE or just SYNC="server1 server2 ..."). The reason is simple: if you have a local portage rsync server and maybe a notebook or something else that moves between networks, it won't be able to access the local mirror from another location. Good would be to check if the server is available and if not just try the next one.
*** Bug 404405 has been marked as a duplicate of this bug. ***
The SYNC variable has been obsoleted by the repos.conf configuration system