You use a stereo jack in the "Ric-o-sound" socket for split, or a normal mono one in the "Normal" one.
The stereo jack is also known as a TRS, for tip-ring-sleeve, with the treble pickup connected to the tip and the neck one to the ring.
You can't use both at the same time, because the mono socket has an integral switch which connects the neck and treble pickups together.
The advantage of the stereo cable is that it's a single cable rather than two; the disadvantage is that you then have to split them (via a Ric-o-sound box, or your own assembly - I think I'm making one today, funnily enough!).
Incidentally, to further contribute to the head-slap, RIC did release one guitar (a particular 620 model) which had dual mono outputs rather than the either/or Normal & Ric-o-sound ones.