A genuine business will be free to take on as much work as it chooses and to work for more than one client concurrently. You should avoid contractual clauses that prevent you from working for other clients during the course of the engagement, as HM Revenue & Customs would see this as a strong indicator of control. See
IR35 ComplianceGuide - Control.
Having concurrent contracts will also help you to illustrate that you are genuinely
in business on your own account (IBOYOA)