Multiple Active Directory problem in Serv-U MFT

Hi, I've defined 2 Active Directory domains in Serv-U Management console. Both are enabled. But when I try to login from Serv-U Web client my username and password are always checked in one of the domains. It does not check both of them if the username couldn't be found in the first AD domain.

I really wonder how Serv-U MFT authentication works when there are multiple AD domains defined in the management console.

Thanks.