You cannot have more than one Primary residence. Therefore, one of you owns a house that you do not live in. Therefore, it is a NPPR and liable for the tax.
However, the following excemption may apply to you, depending on the timing.
Moving home
There is a limited exemption if you are moving home and, in the process, own 2 properties for a relatively short period.
The charge must be paid if you own a second property on the liability date (31 March each year from 2010 on) even where this has been acquired as part of the process of moving house. However, if you move from the first property (your main residence) into the second property not later than 6 months after 31 March, you can get a refund of the charge as long as you no longer own the first property.
To get a refund contact your local authority.