If you want to get started with setting this up, let me know:

: Starters and wild encounters feature entirely unique typing structures that completely diverge from core series rules. Core Features of the Pre-Alpha Builds

The link was buried on page 42 of a defunct ROM-hacking forum, sandwiched between broken image links and "LULZ" signatures. The thread title was simple:

The introduction of Parallel Forms alone makes every battle feel new. Re‑learning type matchups and discovering which Pokémon have been transformed adds an element of surprise that even veteran players will appreciate. The revised storyline, complete with a morally ambiguous Team Azure and the mystery of the forcefield, provides narrative depth that many hacks lack.

If you are looking for a more polished or "definitive" Sinnoh experience, consider these popular and stable alternatives: Renegade Platinum

The gameplay of Pokémon Parallel Platinum pre-alpha is similar to the original Pokémon Platinum game. You'll control a young trainer who sets out on a journey to become the Pokémon Master. Along the way, you'll catch and train Pokémon, battle against other trainers, and explore the Nova Haven region.

Before we talk about the process, you probably want to know what makes this specific hack so unique. At its core, this is a 3DS ROM hack that uses Pokémon Alpha Sapphire as its base. But unlike simple texture swaps, this hack completely overhauls the experience.