In Companies House’s interpretation of legislation, it is not possible to form a company with joint shareholders.
They will typically reject any application to form a company with a joint shareholder – point #8 of our article at http://www.informdirect.co.uk/shares/joint-shareholdings/ has some further details.
Note: As is stated in that article, there’s nothing to prevent shares being transferred from an individual to a joint shareholder once the company has been formed.