Good collaboration between a Scrum Master and a Product Owner is essential for a good Scrum Team and regardless of the position an organization is hiring, the candidates need to understand this.
Both Scrum Master and Product Owner candidates should be able to provide insights into exactly how they would support the other role and, just as importantly, when they would ask for assistance from the other person.
Just as an example, both Scrum Master and Product Owner candidates should understand how they should collaboratively present the product, the Product Backlog and the work to be done to the Scrum Team. By working together closely, the SM and the PO can also ensure 100% open lines of communication which call for easier problem solving and a more efficient process.
The important thing is that the candidate understands how these two roles have a common goal and how they can support one another.