For each image, you will need to make a copy. Convert one to 8 bits with no shift, and convert the other to 8 bits with an 8 bit shift.
The only problem with this is neither image will look good. Instead, I would just do a 2 bit shift on the original image for the avi, and save the original 16 bit image in PNG format.
Bruce
Bruce Ammons
Ammons Engineering