Reciprocal / Locum Tenen Billing

Provider A is going on maternity leave. She hires Provider B to provide services to her patients from 2/3/15-5/3/15. Provider A's last day of work is officially 2/17/15. Both Provider A and Provider B see patients 2/10/15-2/17/15. Provider B sees patients when Provider A is otherwise unavailable (returning patient phone calls/emails, signing/requesting orders, cleaning up before she leaves). If Provider A is billing for her services would reciprocal billing Q5 modifier or locum tenen billing Q6 modifier be applicale for Provider B's services on the same date of service? My understanding of the Q5 and Q6 modifiers is Provider A must be "unavailable". My office has conflicting interpretations of "unavailable". Does anyone know where I can find CMS's definition of "unavailable"?

Thanks!