Per UKBA
We need 6 letters or other documents addressed to you jointly or in both your names. If you do not have enough items in your joint names, you may also provide items addressed to each of you individually if they show the same address for both of you. Examples of acceptable letters and documents are listed below. They must be originals.
The dates of the letters or documents should spread over the whole 2 years. They should be from at least 3 different sources.
Please give an explanation on a separate sheet if you cannot provide 6 items; if the documents are not addressed to both of you; or if they do not cover the 2-year period. If you and your partner lived with relatives or friends for some or all of the 2-year period, please provide a letter from the relative(s) and/or friend(s) confirming this.
If you did not live together for any part of the 2-year period, tell us the reasons for this and whether you stayed in contact with each other during this time, and provide any relevant supporting evidence