How to create shadows by overlapping one shape on top of another identical shape

[Poto]

I want to make the effect of that shape on the right side of the picture. But I am not sure how I should do it.

[MEB]

Hi @Poto
Drag the shadow shape layer over the main shape layer in the Layers panel to clip it to the main layer (over the label of the layer not over its thumbnail).

[Poto]

Did it. But I feel that the effect of lightning shadows is not quite like the original image on the left compared to

So, here comes the new question. How to draw the perfect shadow?

[MEB]

You have to adjust the grey shape a bit eventually enlarging it and rotating it a bit to better match the image on the right. You can also switch to the Node Tool and adjust/position the nodes of the grey shape individually to fine tune the results. Duplicating and then clipping the grey shape is not enough to fill in the main shape as shown in your screenshot on the right.