Even Charles Brown didn't believe that Gary Smith could simultaneously train two powerful Pet Beasts.
Not to mention, just proficiency in skills alone would take up a lot of time. In the future, they might become his left and right arms, but for now, they should be slightly inferior.
By common sense, the first Beast contracted by a BeastMaster tends to be the strongest.
After all, most BeastMasters have limited resources and energy, and would choose to focus on training 1 or 2 Pet Beasts. Even if they establish contracts with other beasts, the first pet beast is still considered the core for developing new tactics.
This is exactly the case for Joseph Goodman, the Thunderwing Snake as his third contract beast is realistically just supplementary. His other two are Wind type and Water System Spiritual Metamorphosis stage pet beasts, pursuing the path of the Storm Spirit Puzzle.