Would like to check what's your opinion of doing 99-1% tenancy in common split? The advantage is that you pay less stamp duty for decoupling. Any downsides to a 99-1% split?

It depends.

If the 99% pax have financial ability to take up the new loan, then it's feasible to do the decoupling.

Beside the 1% pax, shouldn't use so much of their CPF. Otherwise, the amount to be refunded will cause a negative sale in this transaction.

Assuming 1m property when purchase.

if 1% use 100k cpf,

But when u sell at 1.2m, means u sold at 12k.

But because use 100k already, there will be a negative sale.

This 100k CPF usage has to be top up by cash.