GBA-PK-multiplayer
This is a free mod that will in the future support interactions between all of the 3rd generation pokemon games. Alpha 3 currently allows 4 players to see and trade with each other in the game of Firered or Leafgreen. Updates and more information can be found on youtube, patreon or pokecommunity.
Patreon: https://www.patreon.com/user?u=81688818
Youtube: https://www.youtube.com/channel/UCdXg0-BF9FblZ2GTi3u4orQ
Pokecommunity: https://www.pokecommunity.com/showthread.php?t=484949