The way the story goes... They didn't explicitly fuck up, some dude just though two snakes looked cooler than one and we've just been running with it for a few decades now. There is a story about how it was originally adopted by military personnel who transported medical supplies (along with messages so... it would kinda made sense) but the justification there does feel super cherry picked. Mostly, the accepted reasoning is that the caduceus is just a better design.