In the below blog this topic is discussed and as per that for domains other than Domain in which the SAP System service specific User ID/SPN is defined should trust this domain for this to work.
Configuring SAP SNC without Single Sign-On on UNIX/Solaris/Linux
In the context explained above..
All other domains from where users login should trust Domain B. I am about to test this and will post my findings back.
If any one has already implemented this approach please share your inputs.
regards
Jay