app.selection.words gives you a reference to the word.
You can use story.words.previousItem(word) to get the previous one.
It's probably faster to work it out based on character indexes though.
Something like this:
var sel = app.selection;
var story = sel.parentStory;
var curWord = sel.words;
var curWordIndex = curWord.index;
prevWord = story.characters.item(curWordIndex-2).words;
You can then use move to move the words around, but you will probably need to deal with the spaces as well...