As an easy temporarily solution or placeholder this should work: the first problem is, are the hands always in the exact same distance? Maybe not, and since a spear cannot be stretched and squashed, you first have to decide which hand is the hand holding fast to the spear and not move around.
Then I would parent the spear to that hand. The spear could then get a Track To constraint with the other hand as target (or an empty parented somewhere near the hand) so that the spear points at the right location. So the first hand moves the spear which is rotated towards the second, but might be sliding a bit by the distance between hands.
- Place the spear in one hand and parent it to that bone. The origin of the spear should be at the hand's location, because the origin will be the pivot point for the spear's movement.
- Now give it a Track To constraint and use the other hand as a target. Figure out which track axis works so that the spear is pointing in the right direction. If this is too rough, maybe parent an empty at a better location close to the other hand and track the spear to this empty.
- Now the spear should be animated with the character.