Should a latecomer make two opening takbirs when catching the Imam in rukoo' or sujood?

The opening takbir (Allahu Akbar) is a pillar of prayer that the worshiper must perform while standing upright. If the latecomer joins the Imam while he is bowing, he should say the opening takbir, and then say a second takbir for the bowing. Some say that the opening takbir alone is sufficient for him. However, if he finds the Imam sitting for the tashahhud, some scholars say he should say takbir for sitting, while others say he should not.