It could be the bike is just part of the theming and all riders will sit in the sidecars.
While this announcement makes the attraction official, the rumor that a Hagrid themed coaster, including ride vehicles that resembled a motorcycle and sidecar, had been previously rumored and now those rumors are confirmed to be true.
It’s an interesting design for a ride vehicle as it implies that one rider will sit on a motorcycle in some way while the other would occupy the sidecar.
This has the added benefit of making it worth riding the coaster more than once as the experience might be slightly different depending on which car you sit in.