Well, that all depends...
Just to point out, Plusle and Minun will NOT have the same Sp. Attack as Pikachu with a Light Ball when those two are together. I think their Sp. Attack only is multiplied by 1.5, not by 2.
Because both Pok?mon are some of the faster Pok?mon out there (probably faster than Pikachu), they have a chance to dish out some crazy attacks before going down. I remember in Colosseum I had a tough time beating a Plusle and Minun team. (The fact that most of my Pok?mon were weak against Ice didn't help...)
Either they could buff themselves up and then switch to a different team with Baton Pass, or one will use Rain Dance, and the other Thunder. Considering that both are Electric types, Thunder is 1.5 times stronger. For those who are resistant/immune to Thunder (Dragon, Grass, Ground), one or both could have Hidden Power which would be stronger against those types (Ice, Water) If needed, they could use Helping Hand, especially to attack those who love using Earthquake. Being so fast, not many Pok?mon can outrun them.
But enough about them! I like using Flygon and Groudon together since both can know Earthquake, Fire Blast, and Solar Beam. (This team had to be continually recalled just to take on said Plusle and Minun team...) But since I know this isn't a good type matchup, I'm gonna have to figure out a better team that involves Starmie...