So I know you can use Calyrex Ice or Shadow Rider, but can you even actually use Spectrier or Glastrier? I already finished the Calyrex part of the story, and I can't use Spectrier even though I picked the shaderoot carrots place. So can you even use them? Or are they just there to be a part of the storyline?
The Reins of Unity... separate Ice Rider Calyrex or Shadow Rider Calyrex into its constituent Pokémon.
You can use the Reins of Unity, obtained through the main story, to separate the two.
Source
Hope I helped!