Tenants can apply for a joint tenancy if they wish to add a spouse, partner, or a live-in carer to the tenancy.
In both cases, proof is needed that the applicant has been living at the property continuously for the last 12 months. Utility bills are the normal way of doing this.
In some circumstances (like a relationship breakdown) we will agree to change a tenancy from two tenants to one. This depends on personal circumstances, and you should seek further advice from your neighbourhood housing office.