I agree that in 3.5 penalties are unnamed and they all stuck when coming from different sources but not when coming from the same source, as a general rule.
You absolutely cannot cast the same curse twice and give a -12 constitution for example.
IMC I let different curses be cast on the same target, so for example with 3 consecutive bestow curse you could give someone a -6 to strength, dexterity and constitution at the same time, but otherwise one of our other DMs has banned it in her campaign because it was effectively quite powerful.