This is not the case. The cards on an account all have different numbers. If one is compromised, they only cancel and reissue that one.
Saying that, with the fee free offer it makes sense for a couple to have one account each (with supplementary cards on each other's accounts, appropriately marked).
Apologies, I stand corrected.
My experience with CBA many years ago is that they cancelled my compromised card only, leaving my wife's supp card alone.
I don't know if Citibank is any different or indeed if CBA still does the same thing.
Do you know if the CBA supplementary card had the same number? My understanding was that supp cards on the same account always had the same number and would therefore automatically be stopped/cancelled, hence my comments.