Staying Logged in Across Subdomains

How can I make it so my users stay logged in to the site if they transfer over to a sub domain? I have separated my site into two subdomains as I didn't see any way to have two different eCarts on the same domain that can be processed by a single checkout, plus it gives a more focused site for SEO, though it would be nice to have the one checkout if there was a way and it would be saved across the subdomain.

www.cis-prints.com
printing.cis-prints.com (under construction now)

Thanks

I'm not sure why you would need to have 2 different eCarts applied that would be processed by one checkout. You would just have one eCart applied and the add to cart buttons would both be adding to the same checkout in that instance.

To ensure that you are logged in across subdomains, you would have to ensure that the session variables are preserved across these subdomains. If the session variables are retained, the login should be retained as well.

The two carts have different information as one is finished works and one is printing for others. I have devised a way to do this with some custom coding so it can display the different types of products in the same cart correctly.

If the session variables are persistent across the two subdomains then the log in should be retained across the subdomains. There are some session variables that are set when the user logs in and used to check what level of login is used. Are you able to get the session variables to be retained across the subdomains?