Not completely surprised I found a bug in my own code.
Working out of my spam button "F", where HKN had all the priority casting code. LavaB was my highest prio cast. But I wanted it on a button which is easy to each. Originally this was "F" which had Lightning Bolt as default cast macroed and bound in game.
But for the slave this in game bind kept overwriting the HKN code. This made the slave cast LavaB less times then the main. To get passed this I just swapped Lightning Bolt and LavaB as the ingame keybind for "F" on both chars.

My suggestion in my earlier post using F as outbound and 3 as inbound didn't work, as "3" had no refence to the assist macro and therefor the slave never assisted attacking.

But with these changes it now works again. I might have to redo it a tad since it will prio LavaB even if it doesn't have FS up. But that should be solvable with a macro.