It doesn't take long for the weather to get colder in the alps and in September it does get quite cold. If the weather is overcast i wouldn't recommend going to the top of any cols, very cold on the descent. It is manageable over 2 days easily, I climbed the Croix de Fer/ Glandon and Alpe D'Huez last week and if taken at a reasonable rate they are very manageable, have no fear about equipping a triple chainset or a 32 with a compact it is worth it for the reduced effort. I would suggest an easier col on the first day near Bourg D'oisans. Have fun!

Eric