he means that you should be able to daisy-chain them or wire them each directly to the same input. something to avoid, for example, would be to wire the second joystick to the NC tab on the first.
but then again, i dont understand why people still insist on putting separate 4-way joys, especially 2 or more, when there are very nice switchable joys available. why not pick up a t-stick or two when you shop for an i-pac or omnisticks when you get a keywhiz? just boggles the mind..