In 13w01b, when one hopper was placed above another, items would always go first into the lower hopper, then into the block the upper hopper was pointed at (assuming the lower hopper was full).

In 13w02a, when the upper hopper is pointing sideways, half the items will go into the target of the upper hopper, and half the items will go into the lower hopper, except when the lower hopper is powered and pointing sideways (as shown in the screenshot).