You can refer to your statement, there's some sort of account number on the page 1 or something.

For my case the account number was my visa vox. So all these while, i've been paying to my visa vox (AFTER cash rebate) only.

and the amount still populated correctly for both of my cards. so, yes, you can just pay to 1 card, doesn't have to be both. not sure about other banks but in this aspect uob makes it more convenient for me.

PS: i've always been curious what will happen if i pay to my mastercard instead. but haven't tried.