This is a small tutorial I found while browsing a while back on liking both hands to a prop. Don't know if it works as I haven't tried it but hope it helps.
Can't remember where I found it though

"LINKING BOTH HANDS TO A PROP WITH MOTION CAPTURE 
 
  This tutorial shows you how to link one hand to a prop that has been parented to another hand. An 
 essential technique for motion capture sequences where the imported motion capture is driving the IK 
 of both arms and hands. This is applicable to sequences which require both hands holding an object 
 such as a gun,axe, sword, staff, etc. To achieve a realistic animation, both hands need to grip the 
 object and not move or slide, an effect that is pretty much impossible without using the following 
 technique.
 
 This tutorial includes a sample motion file in Poser .pz2 pose format, and 2 animation clips
 showing Michael 3 with a rifle parented to the right hand, before and after the technique is appled. 
 
 
  Essentially, the objective is to parent a prop to the arm and hand which will be driven by the imported motion 
 capture, parent a prop to that hand, delete the keys for the other arm and hand,and link that hand to the prop. 
 This technique works best where the motion in the key cleared arm
 isn't too extreme.
 
 For this tutorial assume you have a motion capture BVH or pose animation of a man walking with a rifle. The effect 
 you want to achieve is both hands gripping and staying fixed to the rifle as he walks.
 You import he BVH and parent your rifle prop to the right hand. Problem is, the left hand and arm are
 driven by the motion capture and are out of sync with the motion.
 [ Note: If using this technique with a .BVH file. It is advisable to import onto the character and
 save as a .pz2 pose file before proceeding.]
 
 Follow the these steps exactly as described.
 1. Load character.
 2. Load animated pose .In the animation palette, select and delete
  all keys for both hands and fingers.
 3. Load Prop
 4. Adjust and pose the hands to fit the prop at frame 1
 ( The sample motion file already has the hands posed and aligned with the rifle.)
 5. Parent prop to right hand. ( Prop will follow motion of right hand during sequence)
 6.Turn on IK for left hand.
 7.Select Change Parent under Object menu, and choose prop as parent.
 8. Select and delete all keys for the left arm, forearm, shoulder hand etc
 
 Wala!!
 The left hand is parented to the prop, the prop is parented to the right hand."