First of all, my condolences on your loss.
To answer your question, you can’t have more than one joint account, and it’s unlikely a joint account would be the most appropriate one to have in this situation anyway.
An Executor’s Account is likely to be the most appropriate option, but Monzo don’t offer those. Depending on which bank you use, I believe you should be able to grant access to more than one exector - so you and your brother would both be able to have access.
The best thing to do is speak to the bank your mother’s account is already with and see if they can offer you an Executor’s account (or convert your mother’s existing account in to one).