So you just want white areas on one layer to be transparent, is that it?
Assuming the 'leg' layer is over the 'bone' layer, try double-clicking the upper layer and look at the 'Blend if..' sliders and the bottom. Make sure 'Blend If' is set to Gray.
Drag the right 'This Layer' slider to the left slightly, making sure that the Preview check box is checked so you can see the effect. To make the effect smoother, option/alt click the slider to break it in two. This allows a certain range to gradually become transparent.