Apparently they're the only strong trainers post-game.
You cant battle Sina and Dexio post-game. However you CAN battle the E4 (obvious location) , restaurant Le Wow (lumiose city), , Inver (route 18) , and the Battle Maison (Kiloude city) for strong post-game trainers.
Sauce: Google, alot of Google.
Dexio and Sina are first met by the player in Route 4 where they discuss about Flabébé and the Fairy type. Dexio then introduces himself as a Pokémon-voyage veteran who received his Pokédex from Professor Sycamore two years beforehand. In the gate to Lumiose City, Dexio tells the player about friendship and gives the player TM27 (Return). After arriving at Lumiose City, Dexio tells the player to follow Sina to Professor Sycamore's lab.
>Further on, he and Sina give the player the Coastal Kalos Pokédex at Route 8 and the Mountain dex at Route 13. The duo are met soon after at the Kalos Power Plant, disguised as the Masked Heroes, attempting to stop Team Flare. -Bulbapedia Dexio Page and Bulbapedia Sina Page
It says nothing about them battling them post-game. So you can't battle them post-game. :(
Source: Above Links
Hope I helped!!