Two separate installs corrupting both installs

There seems to be a major issue with oSClass installs
I am using a webhost & I installed osclass on two different domains on the same webserver (which use very separate directories on file manager). I also checked the database table. The are also different prefix. The table prefix is also different.
I installed my first osclass install on abc.com . I did not import any cities data. But when I published an ad, I added a city under region east in new York in country USA
Then I installed another install of osclass on xyz.com
Then I imported city list for some countries including USA.  But now the country list shows up on both installs.
Also in usa the country list does not show any cities though it said it downloaded ok.
Also if I change the admin password on one install it also chamges on the other install. So weird.
Why is this happening ?

The reason I want to use two installs is because I plan to use one for sandbox testing

if you are on same domain, cookies/session might be shared causing such problems, but besides that database prefix is specified in config.php of installation. I used to have many osclass installations under one database and always worked OK.