For decades, a second yellow card was not shown. And it is really unnecessary, as the game at hand is unaffected by whether it was a 2CT or a straight red. I believe it was around 1990, when cards first appeared in the Laws, that the showing of a second yellow was added so participants would know immediately if it was a second cautionable offense or a straight send off offense. Perhaps the thought process is that no one cares if the touchline is getting a second caution or a straight send off? But having a separate protocol is bewildering to me.