We didnt have anything other than the rent contract and bank account info, as you know visitors here are not allowed to have many official proofs such as credit cards or medical papers, but according to their own check list what we submitted should be sufficientSo sorry to hear about this added stress. Hopefully we can do what we can to help you out and get you past this extra hurdle.
Are they asking for proof of cohabitation from a specific date range? I've seen them ask for specific months/years in other cases, which is why I'm asking. Knowing a date range might help us all in figuring out what kind of proof might be most useful. I know you mentioned that they asked for the tax returns from two years, but I'm trying to figure out why the agent would ask for these (and employment history). I'm guessing you two covered these things in the application already, so not sure......
If I may ask, what other proof of cohabitation did you provide besides the rental agreement and joint bank account? I'd think this would be enough, but you never know with IRCC. I know my wife and I were having a bit of trouble with this part of the application too since we had little in both our names and the same address, so what we did was provide screenshots of social media posts, screenshots of text messages between me/sponsor/sponsor's parents/landlord that showed I lived with my wife, mail from friends that happened to have both our names/address on it, etc. In isolation these things wouldn't prove too much, but with the rental agreement and bank account, it all hopefully meant a bit more.