User Mappin between R3 and EP

Dear All,

we have separate servers for R3 and EP,

There are some login id'a, who have access in R3 as well as EP, but there user names are different in R3 and EP,

I came to know that, we can login in R3 and EP with the same login ID, Can you please let me know the process, how it works..

I think, its possible through "Single Sign On"....

Please let me know the process, How it works? and How to map it?

Thanks in Advance................

Regards

Vamsi.