First, the contrast between (10) and 11, and also between (12) and 13shows that the second wh-phrase in the apparent multiple sluicing construction is required to be PP, but it cannot be DP: The contrast between PP and DP as the second wh-phrase in these examples is understood as saying that only the former can undergo rightward focus movement.
it has often been noted that DP can be shifted rightwards when it counts as 'heavy'.