*shrug* Damned if I know. Would have to look up the rough dates of adoption by both, though given that both were in contact with the roughly same steppe nomads they may also both have copied the basic idea - in slightly different versions - at different times.

OTOH, cut-outs in shields by themselves were nothing new. Just take a look at the "violin"/"Boeotian Eight" oval shield with symmetrical side cut-outs used by both the Greeks (before the adoption of the aspis) and the Persians, and earlier still IIRC by the Mycenean spearmen.