First of all, you may want to try toggling "Additive Overlay Blending" on (press the "Options" button in the "Layers" panel. With this mode, regions where green and red overlap will appear a salient yellow. This allows people to visualize the regions where there is only one of the two activations, as well as regions where they are both seen.

If you want to do something more complex, you will want to create your intersection image using a simple script. With Matlab you could use my scripts: use the nii_thresh to threshold each image, and then use nii_mask to find the intersection:
  https://github.com/rordenlab/spmScripts
You could also do this with a Python script. If you are not familiar with these, you could ask on the NeuroStars web site.
  https://neurostars.org