Just to add to David's answer in case you are not familiar, you need to make sure that the proxy pass you use either preserves the host or maps to a host that is the same as the VIRTUAL HOST you configured when you added the new portal instance. If you send something through that does not match any of the Virtual Hosts that Liferay has in its VirtualHost table, then your request will not fail, it will be routed to the default instance.
