So almost every other double battle in the history of the games allows you to battle on your own with two of your pokemon, but this one suddenly says "no, you aren't allowed to use two pokemon, go find a partner"? Makes perfect sense.
The only way this would work is if they dont let you have more than your starter at that point. Either because there is no tall grass (but then why are all these trainers hanging around?) or because they dont give you pokeballs yet. And that would just be annoying.
It wouldn't just be a random double battle, it would be specific people that want to have one. Heck, it could be some of your other rivals. --- "You really think 4/10 people who go hiking fall off cliffs?" -TrevorBlack
Yeah, that's a good idea. We could get a friend rival or a douche rival to choose from, both groups are happy! --- Rattergun, you are truly a hero for our times. - Recoome_is_god http://i.imgur.com/dUNdc.jpg