I just tried to create a macro that casts a Frozen orb and after that a Blizzard.
It worked for a couple of hours but later on it didnt work anymore.
I dont use PVP talents.
When Frozen Orb hits, i can cast a Blizzard instantly, it even showed me the glowing Border on my macro.
Unfortunately castsequences are inherently buggy. If it was working and then stopped without you switching talents something just broke. ReloadUI will typically fix it.