If you have separate accounts, can you at least show that joint bills are being paid from these accounts? For instance, if the rent/mortgage comes out of an account in your name, and the power bill/internet/whatever comes out of your spouse's account, that at least still shows you are both contributing to bills and expenses for the marital residence.
Also strongly agree with the other suggestions to provide proof of anything else you can get your hands on, such as insurance with both of you covered (health, car, home/renters/etc.), documentation of having each other as beneficiaries (life insurance, retirement), any utility or bill that has both names (phone bill?), etc. Also think outside the box - do you have joint memberships (gym, Costco)? Do you have pets where the vet would have a record that you are both the owners? Any mail addressed to both of you, or invitations addressed to both of you (wedding invite, baby shower, graduation)?