A biomechanist needs qualifications in both mechanics and anatomy. It is possible to become a biomechanist starting from either direction: i.e. a degree in engineering and additional qualifications in biologial sciences, or the reverse. Either way, to be a biomechanist you need to be a masters level. Typically you will be working in a research capacity within a university, a clinical setting or a sports science.
It is quite a glamorous profession in that those with only one discipline: engineering, biological sciences, sports sciences, will not have the wherewithal to tackle the work that you, the biomechanist, can do.