It depends how the house is owned. It can be either a joint tenancy or a tenancy in common.
If it is owned as joint tenants each owner owns the whole with the other, so when one dies the surviving person is the sole owner of the whole and there is no 'share' to be left in a will.
If it is owned as tenants in common, then each owner owns a share - usually half each. This share can be left to whoever they like in their will.
You need to look at the title deeds to the property.
The Proprietorship Register will show the names of the people that own the property and, if it is held as tenants in common there will also be the wording "No disposition by a sole proprietor of the registered estate (except a trust corporation) under which capital money arises is to be registered unless authorised by an order of the court". If there is no such wording in the Title Document then you are almost certainly joint tenants.