I don't know how many characters you are boxing but first of all get a mouse broadcaster. The best way to do it is to have enough computers (I use octopus) to broadcast clicks to every single client. I use the same size monitor resolutions for my 3 box as well which helps. If you set both of your characters camera in the same view it will get it a little closer to on top of your mains. You can look up how to set your view to the same on both characters. The last thing i would do to get your volleys right on top of each other is to set a 2ndary keybinding on your alt accounts to move forward.

So with this set up you would run up to your mobs
Press your forward button to get your followers close to on top of your main
Set Camera view on all characters to the same (If you haven't moved anybody other than that all of your screens should look the same)
With octopus i have they same key that does all that turn on mouse and click broadcasting.(Mouse broadcasting just so i can see if i'm hitting the right spots on my other screens)

Then from there press volley on your main character and do as you normally would. It doesn't always work, and you have to make sure your pets have agro or it messes everything up.

The main goal is just to get all of your screens as close to the same as possible so that when you broadcast that one volleyclick, it is in rougly the same spot.