There are several ways to accomplish this:
1 - Create one article per page. The article will have two vertical pages then using alternate layout you make one horizontal spread.
2 - Create one _h and _v articles. You will have to take great your spreads and single pages.
3 - Use liquid layouts to automatically stretch and fit your sing page to a horizontal page ( this will not give you spreads).
Some of the the problems you will encounter are:
1 - Doubling the size of your folio (possibly rendering it unusable).
2 - Method two does not leave you a 1:1 relation ship (2 verticals = 1 horizontal). Navigation links will be a programming nightmare (page 10 vertical will page 5 in the horizontal [but doable]).
3 - Method 1 simply makes a very large and unnecessary file structure.
I've been down this road as my client was insisting on this (like Ikea). I was warned by Bob and others to not go down this road. In the end, My client settled on a single orientation format as it is the best solution.