Assuming you have navigation properties in place on the database entities, you should be able to do:
CreateMap<StudentCourse, StudentCourseViewModel>()
.ForMember(dest => dest.CourseTitle, opts => opts.MapFrom(src => src.Course.CourseTitle));
CreateMap<StudentCoursePast, StudentCourseViewModel>()
.ForMember(dest => dest.CourseTitle, opts => opts.MapFrom(src => src.Course.CourseTitle));
Usage:
var courses = _context.StudentCourses
.Where(c => c.StudentId == student.StudentId)
.ProjectTo<StudentCourseViewModel>();
var pastCourses = _context.StudentCoursesPast
.Where(c => c.StudentId == student.StudentId)
.ProjectTo<StudentCourseViewModel>();
model.StudentCourse = courses.UnionAll(pastCourses).ToList();