How can I make a switch activate two circuits, when both are normally switched independently?
The easiest way is to replace the manual switch with a double-pole switch, such as this example. This allows you to switch two independent circuits with the same switch. Connect the original switch connections to one pole of the switch. connect the other switch pole in parallel with the motion detector (extend the wires connecting to hot and switched-hot at the motion detector to connect to the other switch pole). If the motion detector has built-in wires, you can add the new wires to the existing wirenuts. If the MD has screw terminals, it would be best to add pigtails (short wires) from the MD to new wirenuts and then to the existing and new wires.
If you are unsure which terminals are which on the double-pole switch, try connecting just in place of the old switch. Once you have that working ysing two terminals of the switch, use the other two terminals to for the MD connection.