So I'm guessing you'll never be able to control the second Pokémon in a double battle nor be able to derive some helpfull information from it when capturing it.
What kind of pokemon will appear as the second pokemon could be done easily (will probably be added to PPRNG sooner or later), but nature and ability depend on whether the first pokemon can hold items or not, which in turn depends on knowing the location in the game you are doing the encounter in order to know the specific pokemon that appears first. Adding that kind of info becomes a lot more effort for relatively little gain.


