I was thinking we can create a multi-page PDF file which consists of both the vaccination proof and the proof of recovery, and then upload that file. Surely, ArriveCan cannot prevent you from doing that, as it cannot automatically validate all forms of vaccination proofs. So a human has to look at the file. In any case, even if they ignore the recovery proof and later select you for arrival testing, you can always claim you sent it, and that they have it, therefore you fullfilled your obligation to "present" the proof and claim the exemption from arrival testing. If they say you didn't submit it "properly", then you can say that there is no "proper" method defined for such submission. So I would think that legally this would be sufficient defense against any non-compliance accusation.