I stepped through the code usning
email@domain.no;email@domain.no as the send to address and I could see nothing that would prevent that from working.
Have you tried with one email@domain.no?
Does the FROM: address have an account on the SMTP server being used when
email@domain.no;email@domain.no is in the SendTo: field?
This may have more to do with the SMTP Server and the FROM: and TO: emails than Form Master. Anything in the Eventlog?