You can set up the first master page to be Portrait. Then, select View > Master Pages. Select Insert > New Master Page. Set one master page to portrait. Set the second master page to landscape. Go back to the Body Pages. Select the parent subform. In the Object palette, click the Subform tab. From the Place list, select On Page > [page], where [page] is the name of master page 2. Your form must be set to "flow" content (and therefore is a dynamic form). See "Example of assigning a page layout to an interactive or static form design" in the help.