Debug/testing aids -- none of this is meant for players.
You wake up in a dark room, confused. Your body aches in a way that says you've been still for far longer than any one night's sleep. The air is cold and smells of dust. You don't recognize this place. You don't remember how you got here.
You now hold two runes. Tap Equip below and select both of them -- they'll fill one of the four boxes in your hotbar, in the bottom-right corner of the screen. Whichever box lights up is the one Cast will use.
That hotbar box now holds both runes together. Face something and tap Cast to bring them into effect. The sealed candle in this room hasn't been lit in a long time.
Build the expression you'll cast with the Cast button -- equipping is choosing which runes are in reach, not a saved combo.