Complex sentence: Rather, it is more likely that an areal feature spread across a then-contiguous Graeco-Aryan--speaking area after early Proto-Greek and Proto-Indo-Iranian had developed into separate dialects but before they ceased being in geographic contact.
Split into simpler sentences: Rather, it is more likely that an areal feature spread across a then-contiguous Graeco-Aryan--speaking area. That would have occurred after early stages of Proto-Greek and Proto-Indo-Iranian had developed into separate dialects but before they ceased to be in geographic contact.