Passing strikes would be most common, where the rider was merely holding/controlling the weapon, letting the force of his mount's movement impart force into the blow.
However, a skilled horseman would be able to control his steed with his legs and verbal commands, freeing both hands.
Think of a more modern figure- the cowboy or vaquero with a lasso- 2 hands required to use THAT- and the infamous Mongolian mounted bowmen who did similar things.
That kind of skill would release the warrior to do a few more things with his hands.
However, a skilled horseman would be able to control his steed with his legs and verbal commands, freeing both hands.
Think of a more modern figure- the cowboy or vaquero with a lasso- 2 hands required to use THAT- and the infamous Mongolian mounted bowmen who did similar things.
That kind of skill would release the warrior to do a few more things with his hands.