Sorry, I don't understand your point.

Why is this ok, and fully functional?
---------------------------------
server { server_name test1.loc;
listen 10.0.0.1:443;
...
server {server_name test2.loc;
listen 10.0.0.1:443;
...
---------------------------------

but this is not?
---------------------------------
server { server_name test1.loc;
listen [2001:xxx:xxxx:xxx::1]:443 ipv6only=on;
...
server {server_name test2.loc;
listen [2001:xxx:xxxx:xxx::1]:443 ipv6only=on;
...
---------------------------------


In BOTH cases, BOTH server{} are listening on the same address:port, differentiated by server_name for vhost redirection.