Hi,
If you set up your two Master Pages, it is just a case of selecting the design pages and making some small changes to the pagination settings (see Object > Pagination).
The first page would be set to Follow Previous OR Top of Page "PageArea1". Subsequent pages that you want to be landscape as well would be set to Follow Previous.
Then for the first portrait page, set the pagination to Top of Page "PageArea2".
Switch back and forth as necessary.
Niall