Aren't forests isolated from each other? Or can they have a cross-domain-like thrust relation, but now on the forest level?
Yes, they can: How Domain and Forest Trusts Work: Domain and Forest Trusts
For QlikView services to be able to access/query another domain controller, the service account from the domain where QlikView server is running must be accepted/trusted by the other domain/machine. Ask your sysadmin about required trust relations.