Actually, no. You cannot speed up egg hatching process with more than 1 pokemon with the ability.
In Pokémon Emerald and all subsequent games, it also halves the number of cycles it takes for an Egg to hatch. Both the Pokémon with Flame Body and the egg must be in the party in order for this to work. Magma Armor also has a similar effect. However, this effect will not stack up if multiple Pokémon with Flame Body and Magma Armor are in the party.