You can do it fairly easily with the advanced motion lab. Make your animaton the way you normally would, no special setting or anything, except make the animation twice as slow as you normally would. For example, if you wanted a keyframe at 6 seconds, make one at twelve seconds instead. After you've done this, enter the advanced motion lab. You'll see a list of all the objects down at the bottom. Click the name of any one and it will display the object's properties. Click the first one, which shuld be position. You should see a straight line graph in the graph window. Click the button for the single parabolic graph.(it looks like a hump, 5th from the left side) Now yout object's position will start and finish at the halfway point, and then go back to the start place again. Select this graph for all the components of all your objects.