如何跨两个不同的CI CodeBase共享CodeIgniter会话?

I have two CI Code base say projectX and projectY (Two Different Code Base and yes CI systems files are also separate but same version), Where ProjectX is the main site say http://projectx.com and ProjectY is http://projecty.projectx.com

Now when a user logs into the system (ProjectX) and clicks on some link that takes him to ProjectY. I want the user gets logged into SystemY with same session that was activated when he/she had logged into SystemX.

So my question is it possible and if yes then how or if 'NO' then what are possible ways to accomplish this as i don't want to show one more popup for login

Ok first of all thanks to @LuckyBurger for the link he has given in comment and yes that is a perfect solution for the problem all you have to do is to change the config file in both the code base with same values for session config array