Can't you just put the 2nd chart "on top" of the first one and set the transparency to 100% under Chart properties->Colors. You have the same dimension for your x-axis so it shouldn't cause any problems.
I've overlayed quite a few charts, and that's probably your easiest solution. Note that you'll have to use a "static" maximum on the axes to make sure the charts will always line up. Might need to play some other games as well. And you'll only be able to get pop ups from the top chart.
You might also be able to add a fake extra LEGAL ENTITY value, "Average", using a calculated dimension. Then just check the value of the dimension to decide which formula to use. I'm not sure about the details. Just sounds like something that would be possible and might be more robust than merely overlaying two charts.
I need necessarily a line chart, so I'm going to continue to use the transparency, but I find that it's not very evident to maintain (I have 8 charts as this one)
