Two drupal installatiion in different server with single sign on

Hi,

I have two drupal website, one is for internet and other is for intranet . Both are in different server. But once a user logged in internet he can access intranet by clicking on the intranet portal link.But no need to sign on . i want to create a single sign on effect.How can i accomplish this. Please guide me to solve this issue.

with regards,
Don Asok