If you don't have 6 documents in each name, then just supply as many documents as you do have in each name since you've been living together and explain that it's only been a few months and you don't have many documents yet.
The FLR(M) application is a catch-all for all situations - some people will be applying having never lived together so won't have any mail yet, others will be applying as unmarried partners where one of the requirements is that they have lived together for 24 consecutive months, while some will be applying for a visa extension after already spending 2.5 years in the UK on a spousal visa.