My assumption regarding Tyrant guard is they soak up hits instead of the Hive tyrant getting rocked. I'm not up to speed on the Tyranid rules, however if the Hive tyrant + the guard equal 1 unit then your looking at taking wounds off the majority armor save. So in a sense if you have 2 guard they are the majority armor save so the wound is taken off them. However, you should really read the codex as i'm sure there are additional rules concerning Tyrant guard.