Does any fintechs yet offer proper joint accounts?
Pretty sure Starling does
What do you mean joint account for business ? You can get two starling bank account holders access to a limited company account.
Monzo does, you both need to be Monzo person account holders and then you can create a business account, one person creates the account and invites the second person.
They also have joint personal account as well where you invite someone to create a joint account with you.