Segments automatically eraised when new overlapping segment added

I am trying to use segment editor to create a body trunk segmentation from my CBCT images, in which I need to use logical operator to manipulate multiple segments under the same segmentation object. However, I don’t know why when I create the second segments using ‘threshold’ effect, the overlapping region between the first and second segments would be erased from the first one. How can I allow the overlapping to exist before I do the logical operation? Thank you.

Please read the “Masking” part of this document: