As others wrote, there are two different things here:
- Joint account: two or more persons own the account and the money or are collectively liable for any negativ balance.
- Power of attorney: another or multiple other persons have access to the account to withdraw money. These other persons don’t own it nor are they liable for negative balances.
At least the second model should not be a problem.
The first one could be an issue regarding tax declaration, as the money is owned by multiple legal entities (aka person).
The second one isn’t, as the money is only owned by one legal entity/person and therefore it’s clear that this person needs to declare it.
If you now say that spending someone else money is a problem tax wise, I say no, but obviously the owner and the spending party are required to properly declare it, if applicable at all.
As a married couple this probably is a small issue as it’s daily business. Many couples support each other financially.
For unrelated persons, the owner makes a donation to the spending person, if no labor or the like is expected from that person in return (aka employee). This donation as well can be a transaction liable to some form of taxation.
BTW: Having a business card linked to a business account owned by that legal entity but the card obviously used by an individual to spend money on company business is a form of power of attorney as well.
But these use cases and the declaration of it should not be the worry of Revolut. It would although require that the proper paper work can be produced out of the App to file to the tax authorities.