Can co-executors informally divide up tasks between themselves, or must every task be done jointly?

In practice, co-executors often do divide up the legwork informally — one handling correspondence with the bank, another dealing with the sale of a property, for instance — and that kind of practical division is generally fine for day-to-day tasks. It's efficient, and courts aren't looking to police every phone call or errand a co-executor handles alone.

Where it gets more complicated is with the significant, final decisions: distributing assets, approving the accounts, or signing off on major transactions generally still need the agreement of all named executors unless the will specifically says otherwise, even if the groundwork for that decision was handled by just one of them. Informally splitting tasks for convenience isn't the same as one executor having independent legal authority to make a major decision alone.

The practical risk is co-executors assuming that dividing tasks also means dividing authority, and later finding out a decision one of them made unilaterally wasn't actually theirs to make alone. Co-executors are usually well served by being explicit with each other, early on, about which tasks are fine to handle solo and which decisions genuinely require both of them to sign off.