How to keep users logged in to multiple sites when using User Synchronization

I have a main site at http://www.sitemasters.com.br and a support site at suporte.sitemasters.com.br. I use User Synchronization to keep user information the same on both sites. However, it seems that a user has to log in again every time they go from one site to the other. Is it possible to keep users logged in across websites?