I own neither Phineas and Ferb nor the MiB franchise, and am not making any money off of this.
There was no actual regulation against it.
Neither organization actually had a rule regarding the species of their agents.
For that matter, neither organization could state, with one hundred percent certainty, what species he actually was.
What they could agree on, however, and all that really mattered in the end, was that Agent P was an exemplary agent.
It remains unclear which agency was first to hire him.
If asked, Major Monogram would swear that they had recruited him first – animal agents were, after all, OWCA's trademark.
Zed would state just as emphatically that it was in fact his own organization that placed the agent in the Tri-State Area to begin with – after all, the scheming of that wannabe Doofenschmirtz and the presence of those worryingly inventive boys were bound to lead to contact with MiB's jurisdiction one way or another, so it was better to have an agent in the area to handle issues and do damage control as needed.
As neither organization is willing to unseal their records so as to prove their claims, the question is unlikely to be answered anytime soon. We certainly won't hear the answer from the agent himself – he's a platypus, after all, and they don't talk much.
In hindsight, they really should have caught on sooner to the agent's membership to both of their agencies. In their defense, however, their uniforms were remarkably complementary – the fedora, sunglasses, and suits were all so quintessentially agentlike that each agency simply assumed the marsupial was accessorizing so as to be seen as properly spy-chic.
It wasn't, however, until the incident with Meap that OCWA and the MiB realized the situation on their hands.
A/N: This is just an idea that wandered into my head due to humorously similar naming conventions. Not sure whether I'm going to continue it - I definitely need to re-watch them both if I do. This is just for fun, I'd love critiques and ideas if you have them: please make sure any critiques are constructive, not destructive.