STSADM.exe -o mergecontentdbs will NOT merge content databases cross web applications

I was working on a SharePoint migration project and one of the requirements from my customer is to move a few sites from a content database hosted in Web Application A to another content database hosted in Web Application B. Immediately, I thought about the new stsadm command "mergecontentdbs" introduced in SP2. Unfortunately, it didn't work as expected! Everytime when I try to run the command, it throws the error "Database not found". I then figured out that mergecontentdbs only works if you want to move sites between content databases that is hosted in the same web application but not cross web application.

To workaround that, I have to fall back to use the stsadm backup/restore method instead. :(