You can't bind 1 key to two actions. Well, it *is* technically possible to do with certain tools out there, but it's against Blizzard's TOS to have 1 key press perform 2 actions and can/will get you banned.

What you will want to do is bind two keys to strafe left & step back on one toon, and strafe right & step forward. Like this.

Z - Char 1 strafe left
Z - Char 2 strafe right
X - Char 1 step back
X - Char 2 step forward

Then you can bind the reverse, maybe to C and V or something, whatever you like.

I personally don't really care about spreading out in approx 99% of the fights so far, so I just tap 1 directional key I want to drop my clones off /follow.