You can't just merge the images. You have to recreate your own based on what was in each of the images you want. You can download both images and re-create the Docker files for each like this:
docker history --no-trunc=true image1 > image1-dockerfile
docker history --no-trunc=true image2 > image2-dockerfile
Substitute the image1 and image2 with the images you want to see the history for. After this you can use those dockerfiles to build your own image that is the combination of the two.
The fly in the ointment here is that any ADD or COPY commands will not reveal what was copied because you don't have access to the local file system from which the original images were created. With any luck that won't be necessary or you can get any missing bits from the images themselves.
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