You have to be careful with them. If they have a strong defensive force, it could easily overwhelm one half of your split force. Once that's happened, you can be sure they'll go about hunting down the rest. They may also split their army too, so then the battle becomes more about how much control you have over two seperate armies. There are plenty of possibilities for what your enemy can do in that situation. But, since they're on defence, you probably can't expect them to stray too far from their base without some leftover base support. This probably wouldn't be too good for a full attack, but like I said, hit and running could probably make it worthwhile. For full attack, my opinion is to keep the force together but attack from a direction they least expect, instead of splitting it too much. The element of surprise with that is probably better in that case than the split army variety.