While working with SSO today I was facing one issue where Request.IsAuthenticate was turning out to false always despite of the fact I was creating FormAuthentication cookie. I was having two sites parentsite.com and child.parentsite.com. The sub domain was facing this issue. I noticed this issue was occuring because the same FormAuthentication cooki names i.e the default .ASPXAUTH one.
Change the form authentication cookie name and try to have unique form authentication cookie names for each domain.Example:
After doing this I was able to fix my issue.