SSO between domains

Hello,

I'm trying to get SSO enabled for web applications in WAS 6.1 and WAS 7.
The apps I'm trying to configure have urls with different domains. I've included the domains in the SSO settings for each app server, but if an app
from one domain tries to link to a secured page in the other domain, then the LPTA keys aren't being passed (I can see this in the HTTP header),
and the web app issues another request to login.
I can talk between secured apps as SSO in servers on the same domain OK, so I think SSO is working OK. Has anyone experienced any issues getting SSO
working in an environment with different domains?

I'm splitting domain names with the ',' separator btw.

Cheers,
Alan W.