Maybe you are confused, but a DMX operator works on blocks of 16 DMX channels across 12 fixtures. (16x12= 192 DMX channels)
So with fixture button one selected, you have access to the first 8 DMX channels, and then when you press the button to switch from 1-8 to 9-16, you then have access to the other 8 channels.
So for two 4ch dimmer packs, address one of them to 1 and the other to 5, and you'll have access to all of the individual channels under fixture button 1.
Now your DMX lights can start at address 17, and once you press fixture button 2, and fader one, that is channel 17.
Making sense now? you have access to all 192 DMX channels the board gives you over 12 blocks of 16 channels in which one set of 8 is there, and then with a button press, they switch to the other half of the 16 channels in that fixture number.