The Sunnah is to offer two sheep for a boy and one for a girl, because the Messenger (peace and blessings of Allaah be upon him) said: “Whoever has a child born to him and wants to offer a sacrifice on his behalf, let him do so, for a boy two sheep of similar type and for a girl one sheep.” Narrated by Abu Dawood (2842), classed as hasan by al-Albaani in Saheeh Abi Dawood. 

The majority of scholars are of the view that it is acceptable to offer sheep, camels or cows as ‘aqeeqah, but they differed as to whether they come under the rulings on sacrifices, so is it valid to share a cow or camel. 

The most likely view is that it is not correct to share an ‘aqeeqah. This is the view of the Maalikis and Hanbalis.
See: al-Mawsoo’ah al-Fiqhiyyah (30/279). 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: It is not acceptable to share an ‘aqeeqah, so it is not acceptable to offer a camel on behalf of two or a cow on behalf of two, so it is less likely to be acceptable on behalf of three or four. The reason for that is: 

Firstly that there is no report of sharing an ‘aqeeqah, and acts of worship are based on following the texts. 

Secondly it is a ransom, and a ransom cannot be divided. It is a ransom on behalf of a soul, and if the ransom is on behalf of a soul, it must also be a soul.  

No doubt the first reason is stronger, because if there was a report to indicate that an ‘aqeeqah could be shared, then the second reason would become invalid, so the basis of the ruling is that there is no report to indicate that. End quote. 

And Allaah knows best.