Question

For example, there is one egg and two Pokemon with Flame Body. Would the egg take 1/4 the usual number of steps to hatch?(1/2 x 1/2)

Answer 1

No, a field ability will be accounted for once, and the effects cannot be stacked by having multiple Pokemon with the same ability. So it will only be half as many steps.

http://pokemondb.net/pokebase/11540/magma-armor-flame-body-question