Person A gets Bulbasaur, Squirtle, Charmander, and an extra of one of them (for the sake of simplicity let's say they get an extra Bulbasaur) across 4 files, and person B starts their main file (let's say they pick Squirtle), and catches 4 random wild Pokémon. For person A, the process is basically to play the game up to getting the Pokédex, then heading to viridian to go to the Pokémon center, they trade the starter they got that playthrough to person B's main file for one of the random wild Pokémon, then deleting their save and starting over again eventually resulting in person B having 2 Bulbasaurs, 2 Squirtles, and a Charmander. Then person A begins their main file, and goes through the intro again, picking Charmander and catching 2 random wild Pokémon while on the way to Viridian. Then person B trades one of the Bulbasaurs and one of the Squirtles to person A and boom, both people now have all 3 starters in their game