You need to press and hold that key (you can change it to some other too if you want, ofc) with one hand - and then click+hold the items you want to split with the mouse pointer and drag them to the new position or on top of another character portrait.
Would be nicer if the "split" key, or mouse pointer wouldnt need to be held down but just pressed or clicked once... or if there was a split button in the inventory that works similar as the identify or repair buttons in the trading window.
