They can be broken across the table edge, but a more precise method is to cut them apart with snips.
A single 20 pin header will make two parts - 4 pins for power and 16 pins for signals.
Note that there is a gap of .1 inches between them so you can tell which end is which.
Plugging shields in backwards is a bad thing. Very bad!
So it's nice they way Nick separated the headers that way. (Thanks Nick!)