All you need to do is go back to effects and click Image overlay again, in the APPLIED EFFECTS window you will see a second IMAGE OVERLAY section, this is for the image you will be adding now, so add the image, set its location and you should now see both images overlaying in the same slide. You can keep going back to EFFECTS and do the same process to add more.
**When in the APPLIED EFFECTS window you will be able to expand or contract the settings for each effect you have used.