According to what I'd previously learned, aspirated stop clusters in Ancient Greek only had a single aspiration, at the end of the whole cluster. The reason for writing χθών "earth", φθόγγος "sound" instead of *κθών, *πθόγγος was convention more than anything else.
However, I'm now realizing that prefixes don't always follow this convention: ἔκθυμος "spirited", not *ἔχθυμος.
Was this just a spelling convention? Or was there actually a difference in pronunciation between κθ and χθ?