Having a single product owner for both teams will limit your scale-ability in the future. If you foresee the need to have more than the 20 people in the future, I would highly suggest looking to SAFe. If not, monitor the efficiency of the PO to answer the amount of questions and ambiguity the teams have in understanding the work. I have seen a single PO be over-worked when attempting to do this for two teams. Perhaps your PO is a rock star, but it's worth protecting against burn-out of this critical team member.