I don't think Casimero and Donaire would fight against each other and I'm sure that the Filipino organizers wouldn't allow it. It's better to have a single representative from each country in a single match. There will be lapses if that happens. Both Casimero and Donaire are making a good name in the industry and most Filipinos support them on their journey.
The Filipino wouldn't want them fighting, but If the boxing council orders a mandatory fight then there's nothing else you can do. Nevertheless, it's a step forward for the boxer's career.
We have seen scenarios like this for the Mexican fighters. They have been fighting each other in the international ring, maybe because there were plenty of good boxers in their country.
This same scenario will inevitably happen for any nationality too.
As long as the Philippines has a good amateur and professional boxing program, it won't run out of good boxers. But the problem is that they are not well known boxing promoters in the country. Once, it was Elorde's son who keep boxing alive, not sure what happen though. But then Manny came along, now that he established MP and hopefully it will help a lot of Filipino break into the US soil. However, we already have the case of Casimero breaking out and then joining Donaire in Probellum, not a good sign to me as to what is going on MP.