It should work and does work for me. My recommendation is for the first site which will be the root admin site do not add any host names. Any un-specified host names will resolve to this site.
Add host names as needed through the web ui for secondary sites. So if the first secondary site is host www.secondsite.com then add that host name.
I do not know what you mean by " When I change the index side..."
It should take effect right away as long as your IIS is responding to all requests for the ip address. Though occasionally I have seen issues where it did not work until recycling the app pool by typing a space in Web.config and save it.
Hope it helps,