It means that at the moment they own their property as joint tenants. That means they each own the whole and the survivor will inherit the whole thing on the death of the first one to die.
They are converting it to a tenancy in common which means that each will own a half of the property.
You can't sell a half share in property in which someone is living because no one would buy it. Consequently, the value attributed to it would be next to nothing and therefore reduce his capital for the purposes of funding.