It is better for the person who is praying behind an imam not to say the tasleem at the end of the prayer until the imam has finished the second tasleem. 

Al-Nawawi (may Allaah have mercy on him) said in al-Majmoo’ (3/464):  

Al-Baghawi said: It is mustahabb not to start the salaam until the imam has finished both tasleems. This is the view of al-Shaafa’i according to al-Buwayti, as was quoted by al-Baghawi. He said: Whoever is (praying) behind an imam, when the imam has finished his salaam, he should say salaam to his right and his left. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in al-Sharh al-Mumti’ (4/267): 

The scholars said: It is makrooh to say the first and second tasleem with your imam, but if you say the first tasleem after his first tasleem and the second tasleem after his second tasleem, there is nothing wrong with that, but it is better not to say the tasleem until after he has said both tasleems. End quote. 

And Allaah knows best.