請問與一位未獲批居港非法難民申請二人居屋是否可行，或必需在香港先辨結婚手續?

None of them works.

1. HOS is a public benefit. So unless a person is legally presented in Hong Kong and qualify, otherwise, the Government will deny applications.

For example - a household worker is not qualified for HOS even legally presented.

2. The Immigration Department will not approve anything simply because of marriage. Hong Kong's policy does not actually provide residency to refugees. Instead, it allows them to stay until a placement can be provided. Basically they are always temporary in Hong Kong's' point of view.