Can UOB One Supplementary Card be used to hit the minimum spending of $500 for cashback?

Yes, it can. All transactions on the supplementary card will be posted to the main cardholder's account and thus will count towards the minimum spend and minimum number of transactions required.

Source: I have a UOB One supplementary card​​​

Hi Anon,

If both you and your wife are trying to hit the minimum spending for a single UOB account, why don't the both of you just use 1 card? That would save on any confusion about whether supplementary card spending contributes to the minimum spending (though for most cards, it does). Both of you can add the card to your phones so it'll be easy for you to spend with the card on a day-to-day basis even without the physical card.

Regards,

thefrugalstudent