I live with my Mom and my young daughter. Since I pay the bills with our combined incomes, do we need separate accounts?

By a signer, I mean someone who can sign checks, but is not a joint owner of the account.

Yes, keep them separate -- not only for the sake of Medicaid, but because of liability. If your money is in a joint account, everything is considered as hers. Everything is also considered as yours. So if either one of you goes into debt or is sued, all money will be included. We can't predict what the future holds, so keep money separate. You can be added as a signer to your mother's account or get her financial POA. These things will allow you to sign for her without putting your separate money at risk.

We live together and I pay the bills so I need access to it. I need to keep better records! It is so exhausting.

Yes, absolutely, keep separate accounts. Use a bank in the location where she lives. It may be hard to keep track of things, but in the long run, if she needs a nursing home and Medicaid to help pay for it, you will avoid a LOT of problems with the application process.