No, as a general rule, you cannot. Both apps are run by different companies that sell you ebooks with their version of Digital Rights Management (DRM) on them. This is so the publisher can guard against piracy of their ebooks. Apple's iBooks can only read their DRM, while Amazon's can only be read by their devices or apps. If the ebook has DRM, they cannot be read by another.
I said as a general rule because most ebooks bought at Amazon or Apple have DRM. Some don't, but it's still a safe bet to assume they do.
Sorry, but that's the way the ebook game is being played. All major publishers put on DRM to protect themselves. As a result, you're locked in as to which device can read that ebook - and no other.
Hope this helps.
Marilyn