I am looking to get to BKK this summer. All UA flights I am considering have a stop in Europe somewhere [either CDG, VIE or LHR], then continue to BKK from there. The first segment is a UA flight and it allows me to select seats. The second segment is a partner airline, generally Thai Airways, and it does not allow me to choose seats. Since we are traveling with our kids, we'd like to sit together for these long hauls. Can I simply book this on UA, then contact the partner airline to book the seats? Thanks in advance.