I've broken out the old standby Shaker surface that has been in the LDT and Civil 3D tutorials for as long as I can remember. I like this one in this case because its a nice, steep slope. I'll start the explanation with the work canvas already in place.
I'll create a flat plane and a fill grid, just like before.
Now, using a very similar process, I'll create a cut grid from the same flat plane.
The I'll use the Top Envelope command to add the fill grid to finished ground. Before this step, Finished Grid = EG surface. In this example, the fill zone is a fairly small area on the lower left corner.
Finally I'll use the Bottom Envelope command to add the cut grid to the Finished Grid. Before this step, Finished Grid = EG surface + Fill Grid. The result is the combination of cut and fill applied all at once, just like you'd expect.
I tried this out using the Add Fill and Remove Cut custom commands and they did speed up the process a little but it was still two separate operations for cut and fill. I like doing it first using basic elements to gain an understanding, then using the custom commands to do it quicker but still understand what's going on.