I've been using multiple images for a while now. Use multiple og:image meta tags like this:
Any pages that have already been shared (with one image) can be updated by loading the link in the debugger/linter here: https://developers.facebook.com/tools/debug/
The debugger/linter will not show a preview of all the images – it just shows the last one (I believe below it does mention what images it has pulled through) however it does work.
