Aerodynamics of double winglets
Winglets don't have some mysterious magical wing tip extending property.  They don't add to the total lift of the wing (except to the extent they are canted).  

The best way to conceptualize them is they extract energy from the tip flow and convert it to THRUST which can be deducted from the induced drag.  They are little wings making lift (wing..let, see?), except the lift isn't up (unless they are canted, where there is a small vertical lift component).  They work the way boat sails propel a boat forward and were originally called "tip sails".  

The flow striking the winget on top is curling inboard toward the fuselage so that there is an outboard angle of attack at the winglet's leading edge that creates a lift vector (being 90 degrees to the local AOA) inboard and slightly forward. 

The split scimitar doohicky just flips the effect over to take advantage of unexploited flow below and behind. The flow striking the winglet on the bottom is angling outboard, causing a lift vector to be applied outboard and slightly forward (again at 90 deg to the local AOA).

If you examine most winglets you will see that they aren't just fins that "block" the flow. They have an airfoil cross section because, as I said, they are wings making lift... just not up.

The benefit of the lift being created at the winglet is only sufficient to exceed the thrust and weight penalty of the winglet itself at lower indicated airspeeds where the vortices are stronger.  For this reason you (almost) only see them on high altitude aircraft that cruise at low indicated airspeeds.