Hugo’s said that Monzo’s keeping the option to offer multiple account open.
I’d be surprised if Monzo ever lets you go into your overdraft unless your overall balance is 0. Monzo will then bounce payments if you’ve said no to an overdraft or you’ve maxed your overdraft out. It’s worth remembering that there’s a £20 buffer / unplanned overdraft (with no fees) for all users too.
I can definitely see why you need this functionality for your method of managing money. But personally, I’ve set all of my Direct Debits / subscriptions so that they leave my account just after I get paid so I don’t have to worry about not having the funds in my account. Is there something stopping you from doing the same?