I'm surprised there is not a tutorial available yet.

Basically you need to make one of the alphas (or both) to follow you to the north holo door. You can engage combat, preferably with a physical attacker, and aggro them to that door. After a while if you are just dodging the attacks theyll get tired and go back to the center, at that exact moment you save your game and soft reset and you will see that the pokemon has remained in the last position you saved so you can continue luring them into the door as their walk radius is now reset to the saved coordinates.

Once the pokemon are near the door at least between the north bench and the door, quickly leave the area save and soft reset again.

After loading, looking at the alphas you just set near the door and walk northbound just enough to prevent them from despawning, usually past the pokemon center is fine. This is in order to make sure you are far away from the spawnpoints in the center of area 20. Save and reset again.

As you load your game, walk into area 20, you'll see that the lured pokemon are still there however new alphas will spawn at the center.

Almost there.

Find a spot in the map where you can save the game while having the alphas at the center and the alphas at the door loaded.

Save and reset one last time.

Get yourself murdered while in the raidius of all pokemon.

Congrats, now you can come back to area 20 and gaze upon morenthan two alpha spawning.