How to display two images (ex: PET and CT scans) on top of each other using volume slicer?

Hi all,

I am trying to visualize both PET and CT scan images on top of each which enable me to slide through all the slicers. My images are as 3d NumPy arrays. My objective is to apply thresholding based segmentation to the images. Could you please help me to understand how to do this?

Thank You