Multiple raids can occur at the same time if a player with the bad omen effect walks into a village that is already being raided.
- Go to a village
- Run /effect give @s bad_omen
- The first raid will be initiated
- Run /effect give @s bad_omen again, while the raid bar for the first raid is loading
- A second raid bar will appear
- Two groups of raiders will appear
If you run the command after the raid bar for the first raid is loading, the game will just add waves to the already existing raid.