I'm not sure I understand exactly what you are trying to achieve, does the below cover it?
1.) Have one logon page that is accessible by either http or https from the internet
2.) Allow users from 2 different domains to use this logon page (ie specify their domain somehow)
3.) Get ATM to authenticate the user to one of two AD's based on the domain they selected from the logon page
4.) Send the user to 1 of 4 different possible destinations, abc.com on http or https; def.com on http or https; dependent on whether they hit the logon page with SSL or not
Nimbostratus