Tell me if I'm being a complete sperglord, but in ORAS,

>at once
no

It's a sidequest

postgame you get the other bike

>>27604056
There are specific NPCs you need to battle to advertise for Ridel. In the postgame, once you've battled those particular NPCs, Ridel will reward you with the second bike.

It's a side quest post game, I think post Delta episode. You have to talk to three people (on your bike?). This includes a cyclist in the bottom right of the Battle Resort, an ace trainer (route 119 and up the waterfall, I think, you need acro bike,) and Hex Maniac in the furthermost north part of the desert (Mach Bike)

After talking to them, then you go to Rydel and get both bikes. You don't battle them, you just talk so >>27604281 is talking shit