There are lots of buffers in X, actually, it's just that only 2 are most commonly used. There're PRIMARY, SECONDARY and CLIPBOARD selections, and long-obsolete (but still accidentally used to implement cut/yank rings - a feature long forgotten by many) 8 cut buffers (CUT_BUFFER0 to CUT_BUFFER7).

Check yourself: http://tronche.com/gui/x/icccm/