Two dougas does not necessarily mean two layers 
exist. Sometimes, although the image is on one 
layer, pieces of the image that hold still are 
not redrawn on each douga in the sequence -- only 
the pieces that move are redrawn, then the two 
partial images are combined when the cel is made. 

In the case of the image you showed, I'd guess 
that she stands there and makes some motion with 
her upper body while her lower body doesn't move. 
So the animators didn't want to redraw her legs 
over and over again.

I have an El Hazard cel which is three layers but 
has five douga sheets.

Many Sharp Smiles,
--Drac