Both Feebas would faint on the same turn because on the first turn, if they both use Toxic, they'll both take damage from it at the end of the first turn. Every turn, they will take the same amount of damage if their stats are the same. So, in the end, both Feebas should faint on the same turn, even if one faints first and then the other, but it will still be on the same turn, unless if the stats are different, or if the Feebas are holding items. The battle would be considered a tie, but the game would say that one trainer won and the other lost. But really, if they both faint on the same turn, the battle should be considered a tie. I'm sorry if I got this answer completely wrong, but if I'm right, I believe that's how the battle should go.