I think that 2 ped. means to use the soft pedal and the damper pedal at the same time.
As far as using the soft pedal.... I think that is most up to the performer's discretion, like the damper pedal. There are places where the compose specifically wants you to use a certain pedal, but you must always take into account the piano you are playing on, and the resonance of the area you are playing in. In the end, it is your ears that really decide what you do with the pedals. In the case of Ondine, I tend to use the soft pedal most of the time anyway.... I think in Le Gibet, Ravel specifically says to use the soft pedal for the entire piece.