you can't stack, and the pokemon must be at the front of the party
it cuts the steps to hatch in half (roughly)
the pokemon with the ability just has to be in the party, it doesn't have to be in front. Also it cuts steps in half exactly, no egg has an odd number of steps to hatch IIRC so there is no 'roughly' involved
sorry if I sounded like a jerk about it, not my intention, just currently annoyed at rl stuff atm
If it's worth doing, it's worth over doing! There is no kill like Overkill.