To combine these two methods, you first have to run the image through both detectors. That will give you two sets of bounding boxes. You can check which bounding boxes overlap, and by how much using the bboxOverlapRatio function.
What you do next depends on what you are trying to achieve. If you want to reduce the false positives, then you can take only the cases where both methods detect the person, i.e. where there is an overlap between the bounding boxes from each detector. If you want to reduce false negatives, you can take the cases where either method detects a person. Or you can have some kind of weighting scheme depending on which method you trust more.