Try getting a piece of wood (a broom maybe) that has the same size of a spear and try carrying it using one hand, It's pretty difficult to carry and if someone hits the wooden *thing* you would kinda drop it, if he uses 2 hands to attack, it would be more acceptable and realistic. 
Have you done your research or are you just assuming spears are to heavy to fight with one handedly? Wielding a broom while not having had any lessons in spear fighting is not going to help.
Adarias:
http://www.youtube.com/watch?v=8lJacQLrDx8The move you describe is in it, as well as a bunch of other moves. I hope it helps a bit.
Quote from Wikipedia:
Spears, although apparently simple weapons, have a remarkable variety of wielding methods. They are listed here from most passive to most active motions.
1. Holding the spear or bracing it against the ground, the enemy impales himself.
2. The spear is thrust out with the arms alone.
3. The spear is held stiffly, and the thrust is delivered by stepping forward.
4. The spear is thrust out with the arms while stepping forward with one or both feet.
5. The front hand releases as the back hand and back foot move forward to perform a long thrust.
6. The spear is slid through the front hand, propelled by the back hand. The forward foot steps forward and the back circles the body out of the line of thrust.
7. The spear is thrown, often at a run, releasing when the opposite foot to the throwing arm is forward.
8. The spear is held couched under one arm, allowing a swinging motion as well as a powerful thrust.