Select the units you want for the second order and ctrl-click that second order (with the "No divisions" text). The selected units will be assigned to that order.
If you draw one offensive line after another, keeping the same units selected, you'll get a second (third, etc) arrow from the line you just drew. Notice where the base of the arrow sits. That's the "origin point" for the units assigned to the arrow. They'll start on that stage of the plan once they reach their origin point.
If you need to change the origin point for units (the base of the arrow), while you're holding down the right mouse button you're using to draw the new destination offensive line, you can use the tab key to cycle between the successive offensive lines in the plan as the origin point. Normally the origin point starts on the last offensive line or front line you drew, and when you draw a second one, that new line because the new origin point. But if you're trying to draw a second arrow, and the base of that arrow is stuck on the wrong line, press tab a few times to see it move around and wind up where you want.
You can shift-click on the "go" arrow and select individual arrows to execute just part of a plan, rather than having everything take off at once.
You can Alt-click on an arrow to enter edit mode, in which you can drag the arrow to one side or the other of its default path to fine-tune the course of the units or fill in gaps.