Going by the book each project should have one project lead superior to all other project functions. But I have seen many projects, especially in the technical area, where the project lead was mainly competently leading in professional matters but wasn’t sufficiently qualified as a project manager. Usually the consequence is delivering a good quality but missing the schedule and cost targets.
In complex projects however even a good and experienced project manager can focus his attention either on his job or obtain the technical leadership, otherwise he always is in a role and time conflict. Dependent on where he has his core competence the other area will suffer, especially in case of distress, where he – quite human – will concentrate on what he knows best.
In such projects I have good experience with distributing responsibilities on multiple shoulders. It is important for both “heads” to have an affinity and understanding for the respective other function, but not to have sound expertise in it. Each one provides his core competences to the joint endeavor, ideally on eye level, and the decisions are made dependent on the kind of question (professional or management) in bilateral coordination at the respective lead.
Similar to executing development, construction, assembly etc. of multiple projects often with the same teams in parallel, a Project Management Office can collectively execute repetitive tasks in multiple projects on the project management side as a shared service to support the double head. Or it can even have responsibility for management tasks, e.g. risk and document management, or the commercial part. That frees up space for taking care intensively and generates synergies from learning and cost effects.