I've always wondered about this Oct.
I've asked the question on the boards, and apparently some people did testify that they in fact experienced the state of getting only one h-stab ripped off.
But curiously, I've never seen it happen to me. It's always two stabs at once. Same on both the receiving and inflicting end - I see some pings on the h-stab, and its either both instantly falls off, or one falls off and then the other.
I've experimented with the offline drones that knocking down only one of the h-stabs is possible... but I've yet to see it in the game.
Quite strange I must say..