The packaging for the Apple Joystick (part number A2M2012) states that the
Joystick works on the Apple II Plus. In fact, it will not without an adapter
plug. Apple no longer supplies that adapter, but there is one available from
Southern California Research Group.
Southern California Research Group also supplies adaptors that let you
simultaneously connect two joysticks to an Apple II+, Apple IIe, or Apple
IIGS (but not to an Apple IIc). If a given software package is specifically
written for two Joysticks, it will say so on the box.
For more information, search the library under Southern California Research
Group.