when you fire the spell run a spheretrace from the decal’s world location. add or subtract a bit from the z value for the start and end locations.
if you have bridges that enemies could hide under you may need to do a multi spheretrace and look for enemy actors in the array it returns.
is the camera supposed to be at the average position between the two characters?
I’m not sure that you would need the camera to be a part of the character. the spring arm could be part of the camera bp, and then you could have a function in the camera to move to the average position which is called whenever you move the characters. are you having trouble getting a reference to the camera in your character bp or vice versa?
If it’s a single player game the camera could be the pawn you possess, and control the characters through it by calling functions or sending interface messages. you’d have the camera spawn the characters so you get a reference to them. you could have the decal in the camera if you wanted to.
edit: nevermind the last bit noticed you said local coop earlier