HMRC may ask questions why you would require a separate company. One possible view they might take is that creation of the new company is a deliberate attempt to avoid registration (especially as you are a 50% owner) and compulsory register you. see detailed answer here: http://vatforum.co.uk/viewtopic.php?id=41
I would strongly advise you to seek the advice of an accountant (or even HMRC) before you start trading with the second company (HMRC can and do backdate registrations).