As I understand it, the need for dual citizens to obtain two international certificates (one linked to each passport) stems from two considerations:
- Australia has designed and implemented the system to work like this. The international certificates bear the subject’s passport number so the intent is that you have an additional certificate showing the ‘right’ passport number when flying on a foreign passport. In my case it may actually not be a problem to enter the UK on a UK passport plus an international certificate showing my Australian passport number; but I’m just doing what is intended for dual citizens in the system Australia has chosen.
- The UK’s treatment of international arrivals has switched (like most countries) to determining how you are managed principally according to vaccination status. Acceptable proof of vaccination is key to this: there are much more onerous requirements (quarantine, additional testing) for those who cannot produce an acceptable credential. By obtaining a certificate linked to the passport I’m using I expect to reduce the risk of any glitch – at check-in here and on arrival in UK – with UK Border Force.
I know rather less about US immigration requirements but, as
@Flashback has said, you may want to look at the new rules coming in shortly. See
this link to CDC guidance. For US citizens it looks like proof of vaccination will still not be mandatory but has an effect on the allowable window for pre-flight testing.