Does Monzo offer a joint account?

That’s on it’s way in the form of…

Which is due in the next 6-9 months, according to the roadmap.

Although we don’t know how strict the saving function will be yet, my guess is it won’t be strict at all & you’ll still have easy access to these funds. Stopping spending over a certain amount has been discussed here but as Jack mentions in that thread, it doesn’t seem practical.