Federal probes of the collapsed mortgage-bond boom are shedding light on how Wall Street firms sometimes created securities and sold them to one set of investors, while advising others to bet against them. One firm that was a major player in mortgage securities, Deutsche Bank AG, illustrates a pattern investigators are looking at. While creating and selling mortgage securities to some of its clients, the big German bank was not only advising other clients to bet the other way, but also sometimes doing so itself.