I didn't try yet but I believe it's possible using bar graphs.
You set the xAxis1 and xAxis2 to have the same range (as I understood it's required for this kind of graph). then you set the yAxis1 (on the left) to have a range from 0 to Y1 and yAxis2 to have a range from 0 to -Y2 (where Y1 and -Y2 is any maximum value you want to reach). Then, when plotting graphs, you set wether they refer to the yAxis1 or yAxis2 and they'll be drawn with respect to the yAxis you've chosen - so bottom-to-top for yAxis1 and top-to-bottom for yAxis2.
I'm being a bit busy and I don't have time to give you an example code, but if you play a bit with QCP you should be able to get the desired result :)