How do I get all the items from more than 1 collection?
get_records('Item', array('collection'=>'5,6'));
Would that give me all the items in collection->id=5 and collection->id=6? Or how do I do it?
You can’t do that with get_records at the moment: the collection filter only lets you specify one collection. You could do several get_records calls and loop through them separately or try to combine them (since items can only be in a single Collection you won’t get duplicates).
As it happens, a contributor to Omeka’s codebase has proposed an addition to the code very recently that would add support for filtering by multiple collections at once, so you may see this feature appear in the near future.