犄角之勢

Etymology
Literally: (using the) momentum of (one person) grabbing the legs (of a deer), (while the other person) holds onto the horns (of the deer).

Later on, this phrase was used to describe a situation in which one divides one's military forces into two separate groups so that one can attack on two different fronts.

Idiom

 * 1) to divide one's military forces into two separate groups so that one can attack on two different fronts
 * 2) double-pronged attack; double-sided attack