In the piano piece "Rumänisches Charakterstück, Op.44" on page 8 there is the marking pp along with "due Pedale".
Which two pedals out of the three available are meant to be used here? Using the soft pedal and the sustain pedal would make most sense to me.
You are correct. Keep in mind that the sostenuto pedal was invented in France around mid 19th century, and did take a long time to establish itself (even today usually being an optional feature, with many grand pianos only featuring two pedals). As such for a 19th century pianist like Athanasiu-Gardeev there are basically two piano pedals, so any indication to play both pedals is easy to interpret.
