I have used IX a lot as both under and over insulation in several of my hammocks. At the risk of generalizing, I would say that it's strength is when combined with other insulations. Used by itself it doesn't hold warmth, it only reflects it back. If you are wearing insulating clothing between you and the IX, it can often work fine.
It's biggest weakness is lack of breathability. Too warm and you will be sweaty. So I would say, "No", you can't just get away with two layers of IX and be comortable. I think you would be comfortable in a much wider temperature range with the down.