Yes, it is possible.
A joint tenancy can be severed (cut or split) and converted to a tenancy-in-common by an instrument of declaration. A joint tenancy can be severed by the following:
1. By alienation: which means by selling away one’s share making the joint tenancy a sole ownership or tenancy-in-common. For example, one of the co-owners need money, sells his share to his other joint tenant, such that it’s now owned by one owner, sole-ownership.
2. By partitioning by mutual agreement: means breaking the joint tenancy by way of mutual agreement.
The legal fees can be quite exorbitant (up to $10,000). A Buyer's Stamp Duty (BSD) of up to 3% will apply for the shares that were sold to the other party.