The first pack should have only the first dip switch up and the second should have only the first and third up. This will make the channel 1 for the first and 5 for the second, thus covering all 8 channels you want.
No, you can't separate the two plugs so you get 8 channels out of each pack, its strictly 4 channels. However, yes you can add more packs and "piggy back" or "daisy chain" (which ever term you prefer) off of the DMX outs from the last dimmer pack. You should already be doing this where the DMX line runs from your scene setter to the first pack and then the out of the first pack to the second.