Picture a pipe directly out of the unit needing to go both into the attic and under the house. The unit would be mounted to the left of the pipe. The problem with the Double Y shown on the right is that debris coming down from above, trying to make the turn to the left to go into the unit, will have a strong tendency to drop out of the airflow into the pipe below. If inlets on the lower line are not used often, then the pipe will build up with debris and clog the system. Although the setup on the left might take a few extra fittings, you can see that all the debris will properly flow to the unit.

Secondly, picture the Double Y (on the right) being used where all pipes are running horizontally. There would be one pipe coming from the left and one from the right. They want to flow toward the bottom of the picture. Again it is possible for debris coming from either the left or right to carry themselves through the air stream and centrifugal force would push the debris into the opposite dead-air of the line that is in direct flow rather than making the turn toward the lower part of the picture toward where the power unit is located.