After activating a certain number of SIMs, the Kogan site stops accepting the same credit card details. So you have to enter a different/second card.
I read someone in an old thread was phoning Koganmobile to cancel old SIMs. People told him don't bother - just to activate the next SIM without phoning them - but maybe this is why he did it. Kogan told him they might ban him from buying any more SIMs due to 'misusing the service'.
Does this 'number of times you've used the same CC count' reset to zero at some stage? Or does your first CC remain blocked, meaning it will happen to your second CC too - so you eventually have to phone them to 'unblock' both of them?
(Once they block all your credit cards, you'd have no way to activate the next SIM.)
I think this is one of the times when anecdotal evidence is not beneficial, and direct contact with the mobile service provider is required and preferable.