Unlike David, I'm not going to judge you or your setup not knowing the details. I'm not sure what exactly you mean by "two instance of liferay connected two the same data base". Is it the same database schema (manning the two instances are using the exact same tables) or there are 2 different schemes in the same database?
If it's the first case then, technically speaking, David is right - you have partly configured cluster. You'll have to go ahead and cluster the cache and index as well. Depending on which Liferay version you use you may also need to cluster JCR or whatever you DL and/or IG storage is. This will effectively result in a fully functional cluster - meaning both instances will have exactly the same data (configuration, pages, content, ....). Then you'll have to add/configure a proxy layer (apache, nginix, HAProxy, ...) to direct your end-users to the first instance and your administrators to the second one (eventually administrators can access second instance directly). This way you can create rules on the proxy layer to disable/forbid access to sensitive URLs for end-users. I've seen this kind of setup enforced by systems administrators in many corporations and since it seems they consider it more secure not to relay on application's permissions where they don't have to.
In the second case (different database schemes) then there are two independent portals. It may at first seem that they can by synchronized on database level (replication, db links, triggers, ... ) but this is not the case as Liferay uses heavily caches and indexes to avoid unnecessary db calls. Depending on what you need to sync, you may try to investigate Liferay's remote staging or implement a custom synchronization via web services.
Firmi prego dentro per inbandierare questo come inadeguato.