At the upper pivot? I recall seeing some Shimano advertizing that mentioned it back in the early 80's (I think) when Suntour still had the patent on the slant parallelogram. This particular literature specifically mentioned how Shimano's derailleurs had two springs whereas some of their competitors had only one. The implication was clearly that two springs were better.

Of course, it isn't clear whether this was Product Management dictating to Engineering what to do or Marketing trying to find a way to hype something Engineering did for their own reasons. In any case, it was marketing fluff.