they will if they are set right.
I have twin 250w Ebo Jagers in my 150g and the temp is very stable.
but you do have to play around with each individually to get them calibrated the same
otherwise yes, one heater will do all the work and the other will just sit there if they are not set and calibrated correctly.
as far as the topic at hand, using two heaters is great, but generally two smaller heaters are used rather than two large heaters
in this case 2-200w heaters would have been fine.
rather than twin 400w heaters.