found the answer:
Each person who is party to the property contract must be a First Time House Buyer to avail of the exemption on stamp duty for example. There is not a provision for splitting on a pro-rata basis.
If all buyers are not FTBs then they are jointly classed as non FTBs. Whether or not you will be liable for SD depends on the specific circumstances if the sale (e.g. new builds exempt from SD for any owner occupiers etc.).