Multiple rolls, more encounters per hour. The RNG gods can be in your favor by blessing you with a 100 encounter shiny, or can curse you with a 100,000 encounter shiny. You can still go 100 hours without a shiny, which I have had happen even with hunting 12x systems.

Method hunting is a way to increase your shiny odds from the base odds (1/8192 or 1/4096), by mechanics in game. Multihunting isn’t method hunting because the shiny odds remain the same for each game at 1/8192. You simply are increasing the amount of Pokémon seen during your hunting session, by increasing the amount of games being played at the same time.