How can the total momentum still be zero when two objects are moving rapidly away from each other? 

Momentum is a vector quantity, meaning that it has both an amount (a magnitude) and a direction. When two objects are moving rapidly away from one another, they each have momentums but those momentums are in opposite directions. When you add these momentums together to find the total momentum of the two objects, you must consider the directions of those individual momentums. If the two momentums are exactly equal in magnitude but opposite in direction, they will cancel when you add them together and the total momentum will be exactly zero.