Simultaneous attacks never used to be in WHFB, just 40k. I believe if there is no ASF, but both sides have equal initiative, that is simultaneous now too. You don't neccessarily have to roll at the same time if you don't have enough dice or whatever, but essentially the effect happens at the same time.
So if the elves roll their dice, and kill your ASF champ, then his attacks still get to work before he dies. Or if he rolls and kills all the elves, their attacks will still strike.
To the original question, yes it will only matter if the elves choose to attack the champion who has ASF. If there are multiple models in base contact, you can choose which one to direct attacks on to and whether you split your attacks. Unit champions can be targetted seperately from the unit.