Section 34 of IPC defines what 'common object' is. It seeks to punish the commission of an offence jointly. The essentials as given in the legal principles are as follows-
1. Criminal Act- It means unity of criminal behavior of several persons to do something which is punishable in the same way if all the acts were done by an individual person.
2. Done by several person- Means the criminal act must be done by more than one person.
3. Criminal Intention- there should be unity of purpose or a pre-arranged plan to commit that act.
If all the essentials are fulfilled only then a person will be liable for the crime and not his separate act leading to the crime.
Here, X is not an abettor but jointly liable, the Principle states that when 2 people get together and plan to commit a crime and then some action is taken in furtherance is taken towards carrying out that plan then they are said to have committed criminal conspiracy. X has procured poison which is an act done in furtherance of the agreement to poison Z.
Hence, X is not an abettor any more but a conspirator.
X procured the Poison and Y administered it to Z, both the acts lead to the death of Z and since the common object of both X and Y is fulfilled, therefore, they both will be liable for the act of murder equally. Hence Both X and Y have committed the offence of Murder.