I've done similar by creating TWO charts, and making certain they always have exactly the same dimension values, and line up perfectly, so that it looks like a single chart.
I haven't experimented, but perhaps it could be done in a single chart in version 9 (I'm on 8.5) by making a straight table with a bar chart column. I think it can only do one bar per column, though, so it probably isn't exactly what you'd want.
