From what I understand about IR35 it's also applied on a contract by contract basis.
So you could run one contract Inside IR35 at the same time as another is Outside IR35.
HMRC refuse to fully disclose exactly how they deem a contract to be Inside or Outside but I always go with how much control I get and if I'm billed solely on time or deliverables. Even then there's no certainty, you just have to submit the accounts and hope for the best.