How to add multiple IPv6 address for single host in /etc/hosts?
Well, if you want to do that, just do so :-)
If you add this to /etc/hosts:
2001:db8::1 testing
2001:db8::2 testing
192.168.0.1 testing
then the hostname will have three addresses:
$ telnet testing
Trying 2001:db8::1...
Trying 2001:db8::2...
Trying 192.168.0.1...
telnet: Unable to connect to remote host: No route to host
In this case the addresses don't exist so the connection fails, but you can see that it works.