Why does the A320 right aileron float on dual engine failure?
Assuming it's zero fuel dual engine failure with no chance of powering up the APU, the plane is left with the Ram Air Turbine (RAT).

1. The emergency electrical configuration only powers ELAC 1, one of the two units that control the ailerons, and ELAC 2 is lost (see below).


(A320 FCOM)

2. The RAT powers only the Blue hydraulic system.

Add those two conditions with the color coded schematic below:


(Airbus Training Manual)

And the right aileron will not be powered. The same schematic can be found in the FCOM under Flight Controls > General > Architecture (27-10-20), the one above is clearer though.