A traveller praying with local residents

A traveller arrived [in town] and found the [local] people praying [the] ‘asr [prayer]; they had [already] prayed two rak’ahs, [so] he joined them for the [final] two rak’ahs. [Considering he is a traveller, should he suffice with just praying these two rak’ahs with them or] should he complete the prayer [by praying all four rak’ahs]?

Numerous jamaa’aat in the masjid

What is the ruling about the numerous jamaa’aat [established for the obligatory salaah – taking place] in a single masjid, particularly when some of them pray after the imaam has completed [the first jamaa’ah], and some others intentionally delay [their arrival] until after the imaam [has completed the first jamaa’ah], so they can pray with another jamaa’ah?