Octahedral splitting occurs strong field ligands are coordinated with the central atom. What if we have both types of ligands attached to the central atom as in $\ce{[Co(CN)3(Br)3]^{3-}}$, knowing that $\ce{CN^-}$ is a strong field ligand while $\ce{Br^-}$ is a weak field ligand.
In reality it would perhaps boil down to experimental reasoning, but if it's an exam asking you this, I'd like you to remember a rule of thumb as you'd call it:
- If the complex has a coordination number of $6$, with $3$ strong field ligands and $3$ weak field ligands, then consider all the ligands to be strong field ligands.
- If the complex has a coordination number of $4$, with $2$ strong field ligands and $2$ weak field ligands, then consider all the ligands to be weak field ligands.