difference domain with the same VB

[acyk]

Dear sir

I need your help .

I have three domains for example : vb.org , vb-must-buy.org , vb-rules.org

all link to the same vb and database in vb.org

But when my member from vb.org link to vb-rules.org , system will force log out this member and told him come back after 15 mins .

where I can make the setting , because as I am the admin , I will not logout at anytime .

thanks for your help