I don't think you can do any better than place the bars side by side. If you REALLY must display it this way, I think you'll need to overlay two charts and make sure you carefully calculate the same static max and min for the two charts, and make sure they report on the exact same data points for each year, or use a static min and max on the X axis as well. If they don't like up properly in all conditions, you have problems. I wouldn't do that though. I'd use a line chart with two lines. I think that's much cleaner visually, much easier to interpret.
I agree, using two lines is the preferred method. On that particular graph, I have bars on one axis (current year & previous year) showing revenue and lines on a second axis for conversion. I like mixing bars and lines in this way.
As there is no direct way to achieve the offset, side by side will suffice.