from an experienced 80 rogue, garrote and ambush are on the same global cooldown, so no you cannot get off both before unstealthing. However, if you want to not hit them for white dmg, you can do one of two things:
-Make a macro that goes something like /cast Garrote /stopattack
-Stand behind your target at an angle. Special moves have a wider range in front of your character than regular white attacks, so if you stand at a big enough angle from your target you should be able to hit your garrote while being at too far of an angle to start auto attacks (first way might be easier)