It's possible to combine landscape and portrait subreports, you need a new section for ever subreport.
Right click on one of the section and select "Section expert", from here you can add new sections by clicking on an existing section (header, details, footer) and the Add button on the "Paging" tab on the right you can decide the orientation of you section (Default (portrait), landscape and portrait).
Now create your subreport and put it in the new section.
For a portrait subreport just follow the default creation procedure.
For a landscape subreport create a landscape oriented section and in the subreport set landscape orientation in each section of the subreport.
Page Header and Page Footer won't allow you to change the orientation, so in your landscape subreport suppress those section without drill down (right click on the section to find this option) or Crystal Report will act weird and create a new page for every section with different orientation and suppressing them will prevent this situation.
At the end you will have a situation like this