At the start of the game, around the time of the Mauville Gym, you are forced to make a choice: the Mach Bike or the Acro Bike? Each have their benefits, and each have their uses. However, because the two differ so significantly, only certain areas in the game are reachable by each Bike.
Such is how it was in Pokémon Ruby/Sapphire over 10 years ago. However, there are some areas - such as the Safari Zone, particularly for two TMs - in which both Bikes are required. How do we deal with this conundrum?
Well, cheating is one way, as would applying the logics of Flying Pokémon, but Game Freak decided on another: allowing you to have both Bikes at the same time!
To do so, you must first show three people certain Bikes; due to where these people are located, it will not be until you beat the Elite Four that you can actually own both. The people are as follows:
- There is a Hex Maniac in the northern part of Route 111's desert. Show her the Mach Bike.
- There is a Bird Keeper on the eastern portion of Route 119, though you'll need both Surf and Waterfall to get there as well as the Acro Bike. Show him said bike.
- There is a Triathlete in the Battle Resort area you can speak to; show him some kind of bike, doesn't matter which.