Complex sentence: The idea spread that the rich people do not contribute if they are not required to by compelling them: at the same time, around 355, Demosthenes and Xenophon mention it, one in the Counter Leptines, the other in Poroi.
Split into simpler sentences: The idea spread that the rich people do not contribute unless required or by compelling them to do so. At the same time, around 355, Demosthenes mentioned it in the Counter Leptines and Xenophon in Poroi.