Hi
This issue is now resolved. UME is setup towards the one AD and SPNEGO is setup with two realms, one for each domain with usermapping set to principal only. Each realm needs a service account and an SPN for this account and the portal URL in the corresponding domain.
Of course the browser settings also needs to be in place. The specific settings are described in the SPNego Configuration Guide (attached to note 1488409).
/Jacob 