login on mainsite belogged on other? cookie issuing.

currently i have two wordpress sites on the same database and different prefix using master database of users so i can use 1 login for all site.

my question is is there anyway to issue cookie so if i logon to main blog it automatically logs me on to the second blog?

i know there was a way for previous versions but not sure how to do this for 2.6.2

let me know plz!