Secondary tutors cannot be assigned at learner level, and are instead assigned as a component owner at the learning component level. If you wish to assign a component to a secondary tutor, you may do this by adding the tutor's name, under the component owner section of the component details, when creating a component. This can either be done when creating a component in the programme build, or in the Learning Plan, directly.
The secondary tutor will then be able to view any of their 'secondary learners' in either their dashboard or learner menu, by changing their view to 'Secondary Learners':