On Joint Tenancy, there is an act governing properties in Singapore, as such all properties comes under the purview of the respective act. But as far as religious advisory is concern, the Fatwa Committee is of the opinion that a joint-tenancy asset is equally owned by all the owners, regardless of whether each of the tenants had made any contribution, or the quantum of his contribution. Therefore, the net profit from the sale of the HDB flat must be divided equally (50% - 50%), that is 50% to the 1st owner and 50% to the 2nd owner, after returning the payment that had been made by any parties, if any. Should a muslim wants to give his/her share (50%) of the property to the surviving spouse, a "nuzriah" contract could be made to support the joint tenancy doc.
As this only serves as a religious advisory, it is not legally binding, as such they are at the liberty not to follow the advisory should they decide to do so.
For the full fatwa text please refer to this link: http://www.muis.gov.sg/officeofthemufti/Fatwa/joint-tenancy(2008).html