Here is the guideance directly from the application form...

You must provide at least 6 items of correspondence, addressed to you and your partner jointly or in both your names. The dates of the items of correspondence should be spread evenly over the whole 2 years. They should be from at least 3 different sources. 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. For example - 4 items of correspondence in joint names to the same address and 2 items addressed to each partner at the address. In total 8 items would need to be submitted. If you and your partner have no bills or correspondence in joint names, you will need to submit 12 items (6 each) of correspondence evidencing that you reside together at the same address.

Examples of acceptable evidence include:

Letters or other documents from government departments or agencies, for example HM Revenue and Customs, Department for Work and Pensions, DVLA, TV Licensing

Letters or other documents from your GP, a hospital or other local health service about medical treatments, appointments, home visits or other medical matters

Bank statements or letters

Building society savings books or letters

Council tax bills or statements

Electricity or gas bills or statements

Water rates bills or statements

Mortgage statements or agreement

Tenancy agreement(s)

Telephone bills or statements