If Bella is OK with you being on the account, then I'd just leave things alone until Roberta passes things on to Bella -- but make sure that happens soon.
Having two people on the account doesn't necessarily make things "safer" -- it means there are now two people with the means to steal money.
However, it's probably harder for the president, not having the checkbook most of the time, and since the treasurer (as you say) opens the bank statements and presumably checks them.
In our case, the president is on the account, I'm pretty sure it's in our by-laws, just in case I'm on vacation or out of town when a check needs to be written -- or in case I get hit by a bus. When I was added, the old treasurer and I just showed up at the bank and added me; I added the president later; in both cases, we just showed ID. I'm sure it would be a nuisance to convince the bank to add someone without having an existing signer there to ask that it be done. How does the bank know that those "minutes" are really the approved minutes??