Romeo and Juilet

Well, if you want crack pairings, then Zangoose x Seviper or Groudon x Kyogre since they're supposed to be mortal enemies :>

But if you want something that works, Pika x Chuchu from PokeSupe Plusle x Minun. They were created solely for Double Battles, and they can't really do as much damage when apart as they can when together.
 
Volbeat and Illumise. They would make a nice night sky performance of the play :D
 
Pigeot and Magikarp, have the whole Predator/Prey thing going on.

Also Muk and Garbodor, being complete garbage and all.

Also,
Spoiler:
 
How about Zangoose/Seviper? They are archrivals, but that would fit the story of the play nicely.

This this this.

Or Latios and Latias (not as brother and sister though).
 
Shelmet and Karrablast. I don't know why I thought of those two, but I thought of a good ending (both evolving into Accelgor and Escavalier respectively, and they eventually are killed).
 
Vulpix as Juilet and Growlithe as Romeo. Or maybe Eevee and Vulpix in either role.
 
Ditto and Squirtle
Dewott and Prinplup. They are in different regions yet in the same egg group.
 
Illumise and Volbeat. They are partner pokemon and they just look the part, peaceful and in love. They were the first to come to mind as well.
 
Pichu as romeo and smoochum as juliet. It seems like it would be so cute!!!
 
Zangoose and Seviper would have to be Capulet and Montague since they are the sworn enemies rather than Juliet and Romeo. The lovebirds could be, what, a normal- and poison- type respectively? From their respective 'families' but still in love.
 
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