seq_overlay overlays the corresponding images of the sequences. But as in im_overlay this command does not "fade" images on top of eachother. If sequence1 is on top of sequence2, then sequence1 must be smaller than sequence2 in order to see sequence2. Previous sequences are laid over later ones. The length of the new sequence is determined by the last sequence. If the sequences were of the same size you would have to change the size of the first one with seq_conform. DoubleView is a sample procedure where this is done.