It's for a model painting competition but I don't know if it fits in with any fluff... Opinions? O.o
I will interpret the question as, is there anything in the game rules that would preclude modeling these two figures in combat with each other.
I have not played this one but given my understanding of what's allowed and what's not from various codex-based miniatures games this seems like a plausible pairing. Sorcha is a warcaster (https://battlecollege.wikispaces.com/Sorscha) and Kaya is a warlock (https://battlecollege.wikispaces.com/mkiiKaya) from armies that can and will clash.
You can find in published blogs first person accounts of battles involving both personalities, supporting the view that the two of them actually meeting in close quarters could happen.