It wasn't the same before I did some modification. I revert back to my old config which is different des_key and it is still the same. I'm getting kick out on the 1st RC after logging in to the 2nd RC. I tried also to other browser (IE and Chrome) but issue still occur. I could open multiple RC on a single browser.
BTW, my 1st roundcube is running on https while the other one is running on http.