First, is HotKeyNet actually passing the keybind to both windows?

1. Put the follow macro for your slave on key 1 in WoW.
2. Press 1. What does HotKeyNet say in the "Actions on this PC" window? Copy and paste the message here.

Macros need to be on the same keybinds. This is explained in the post I linked before. You don't list your classes, so I don't know what you're using, but I'll use shamans as an example.

1. On your Main's screen, take the Lightning Bolt spell out of your spellbook and place it on key 2.
2. On your slave, create a macro using either :

Code:
 /assist Merlinbb
/cast Lightning Bolt
or

Code:
/cast [@party1target] Lightning Bolt
3. Put that macro on your 2 key on your slave.
4. Get a target and push 2. Your master should cast lightning bolt, your slave should assist your master and cast Lightning Bolt as well.

There's no screen switching involved. There's no additional keys involved.

And you did not answer my previous question. "Are you moving with the arrow keys?" Your HotKeyNet script passes the arrow keys to both windows, which causes all characters to move independently. You need to move with either the mouse or the WASDQE keys and leave your slave on follow.