hmm...to tell the thruth, i started with sso but eventually gave up because essentially i don't needed it:
I have separate branches per language in my content tree.
This allows me to have fully localized urls and page names, witch i think is a must for SEO.
In web.config i map in <cms><urlMappings> each domain to a branch in the content tree.
My editors don't need to switch domains anymore, because they edit all content on the primary domain
So i realy don't need a cross domain authentication / sso anymore.
The hardcoded admin / ip login is only for convenience within the company network.
But it should definitively be possible to pass the ReturnUrl to the SSO domain and then redirect back to the other domain. You could pass the url in another querystring variable! So you are not limited to two applications.