I've only been dancing for 8 months (although at least 10 hrs every week), and I've been doing international for 3. If you can find instructors who actual know international (not American instructors pretending to), there's no reason not to learn them both simultaneously. I'm already dancing silver in American (not silver syllabus-- USABDA silver competition level), and the only thing that's holding me back in international is that I've just now found coaches who know it! Learning the 2 styles won't hurt you at all-- in fact, exploring the differences has helped me understand each of the dances much better.