It's pretty easy:

Step 1: Make macros to set each player as /focus. "/target Char1; /focus" "/target Char2; /focus" etc. Bind these to easily accessible keys (I use F1-F4)

Step 2: Make all your macros for DPS like this... "/cast [target=focustarget] Shadowbolt"

This will do exactly as you said in your post... you're running along with your "main" and you spot a mob. You target that mob, stop in place, hit the button to cast shadowbolt, and blam.... both locks cast it.

If you're doing more than 2 characters, and one dies, you can just hit one of your F1-F4 keys to change the focus to another player and keep on fighting.