Loop in loop

Hello,

How can I use a custom loop inside another custom loop item?
It seems like both iterations use the same object, so the first loop resets after the second loop renders for the first time.

The example goes like this. I am using the Elementor Pro Posts widget and Ele Custom Skin (not Pro):
– the first CPT is Books and the second CPT is Pages of books
– The first LOOP should show all the Books, and inside each Book item there should be a loop of Pages.
– The first Book with its loop of Pages is shown well. But, the second Book it’s shown with a Page template, and all the loops stop there.
– If I use a standard template, the loops go both well till the end.

So it seems like Ele Custom Skin doesn’t use local objects, but a general default one.

Is there any solution for reseting loops after each use?
Thank you!