I want to create an ebook that will be optimized for both iPhone and iPad, is there a solution to create one epub that will fit both iPad and iPhone.
Another thing, as for horizontal and vertical do I need to create two versions or does the epub can handle it by himself.
When you're creating an ePub document, it's reflowable. It should work relatively well on both larger devices (iPad) and smaller (iPhone). Try to avoid large tables or text running around graphics which may not show up well on a smaller device.
the same goes for orientation ? the device will automatically go to horizontal and vertical ?
Yes, as the iPad or iPhone changes orientation (unless it's been locked), the ePub will reflow for that orientation.