Knife
Cuts a part into pieces along a line you draw. The job it exists for is a panel wider than the machine: it has to come apart somewhere, and this puts the join where you want it rather than where something else decided.
1
Start with the part that has to come apart. Select it first to protect everything around it, or select nothing and the knife cuts whatever it crosses.
2
Drag right across the part, starting and finishing clear of both edges. Hold Shift for a straight line.
3
Two parts now, each one its own shape, ready to move apart and nest. A line that stops inside the part cuts nothing.