Right. Numpad 2 and 8. They do what a controller's up/down buttons do. Which is to move up/down the party list. So you press numpad 2, and your subtarget cycles up the party list. You press numpad 8, and your subtarget cycles down the party list. (Edit: and if you like the way that works, but don't like those keys, you can bind them to something else. They're in the gamepad tab of the keybinding menu.)