If the imam says one tasleem, is it permissible for the one who is praying behind him to say two?

The imam should not limit it to only one
tasleem at the end of the prayer, because the second tasleem is prescribed
and doing it is preferable. In fact Imam Ahmad and some of the Maalikis are
of the view that it is obligatory, and that the prayer is not valid without
it. However the majority of scholars are of the view that the second tasleem
is Sunnah and mustahabb (encouraged). 

According to the view of the majority,
limiting it to one tasleem does not render the prayer invalid. 

If the imam says one tasleem, there is
nothing wrong with the one who is praying behind him saying the second
tasleem, because doing that is more complete and is more on the safe side,
and it avoids an area in which there is a scholarly difference of opinion. 

The well-known view of the madhhab of Imam
Maalik is that the one who is praying behind the imam should say two
tasleems, as will be explained below. 

An-Nawawi (may Allah have mercy on him)
said in al-Majmoo‘ (3/462): The correct view according to our madhhab
is that what is preferred is to say two tasleems. This is the view of the
majority of scholars among the Sahaabah and Taabi‘een, and those who came
after them. This was narrated by at-Tirmidhi, al-Qaadi Abu’t-Tayyib and
others from the majority of scholars. It was also narrated by Ibn al-Mundhir
from Abu Bakr as-Siddeeq, ‘Ali ibn Abi Taalib, Ibn Mas‘ood, ‘Ammaar ibn
Yaasir, and Naafi‘ ibn ‘Abd al-Haarith (may Allah be pleased with them). It
was also narrated from the Taabi‘een ‘Ata’ ibn Abi Rabaah, ‘Alqamah, and Abu
‘Abd ar-Rahmaan as-Sulami; and from ath-Thawri, Ahmad, Ishaaq, Abu Thawr and
ashaab ar-ra’y. 

Some scholars said that the worshipper
should say one tasleem. This was the view of Ibn ‘Umar, Anas, Salamah ibn
al-Akwa‘ and ‘Aa’ishah (may Allah be pleased with them), as well as al-Hasan,
Ibn Sireen, ‘Umar ibn ‘Abd al-‘Azeez, Maalik and al-Awzaa‘i. Ibn al-Mundhir
said: ‘Ammaar ibn Abi ‘Ammaar said: In the mosque of the Ansaar they used to
say two tasleems and in the mosque of the Muhaajireen they used to say one
tasleem. Ibn al-Mundhir said: I favour the former. 

Then an-Nawawi said: Our view is that what
is obligatory is one tasleem, and the second is not obligatory. This was the
view of the majority of scholars, or all of them. Ibn al-Mundhir said: The
scholars are unanimously agreed that the prayer of one who limits it to one
tasleem is valid. At-Tahhaawi, al-Qaadi Abu’t-Tayyib and others narrated
from al-Hasan ibn Saalih that he regarded both tasleems as obligatory. This
was also narrated from Ahmad, and was the view of some of the companions of
Maalik.

End quote. 

Ibn Qudaamah (may
Allah have mercy on him) said: It is prescribed to say two tasleems, right
and left. 

That is because of the report narrated by
Ibn Mas‘ood (may Allah be pleased with him) who said: I saw the Prophet
(blessings and peace of Allah be upon him) saying the tasleem, (turning his
head) until the whiteness of his cheek could be seen, to the right and to
the left. It was narrated from Jaabir ibn Samurah (may Allah be pleased with
him) that the Prophet (blessings and peace of Allah be upon him) said: “It
is sufficient for one of you to place his hand on his thigh, then say salaam
to his brother on his right and on his left.” Narrated by Muslim. According
to another version of the hadeeth of Ibn Mas‘ood: The Prophet (blessings and
peace of Allah be upon him) used to say the tasleem to his right:
‘As-salaamu ‘alaykum wa rahmat-Allah’ and to his left: ‘As-salaamu ‘alaykum
wa rahmat-Allah.’ At-Tirmidhi said: The hadeeth of Ibn Mas‘ood is a saheeh
hasan hadeeth.

End quote from al-Mughni (1/323). 

The view of Imam Maalik, as it says in
Mawaahib al-Jaleel (1/526) is that the worshipper should say three
tasleems: the first to his right, to conclude and exit his prayer, the
second, which he should intend as a greeting to the imam, and the third,
which he should intend as a greeting to those who are on his left. This view
was rejected by al-Qaadi Abu Bakr ibn al-‘Arabi (may Allah have mercy on
him), who favoured the view that the second tasleem should be intended as a
greeting to both the imam and the other worshippers, and the third is a
bid‘ah (innovation). He (may Allah have mercy on him) said: He should say
two tasleems: one to his right which he should intend to conclude and exit
the prayer, and the second to his left which he should intend as a greeting
to the imam and congregation. The third he should avoid, because it is a
bid‘ah (innovation), that was not proven from the Prophet (blessings and
peace of Allah be upon him) or the Sahaabah.

End quote from Mawaahib al-Jaleel. 

To sum up: What is encouraged (mustahabb)
is for the worshipper to say two tasleems, whether he is the imam or praying
behind an imam, and if the imam limits it to one tasleem, it is encouraged
for the worshipper who is praying behind him to say two tasleems, one to his
right and the second to his left. 

And Allah knows best.