Thanks Khatovar. In my first post I mentioned that even when I click on my assist macro on the slave, it will not target my masters' target, I (hope I was) wrong, it could have to do that I really activated this option in Jamba as you mentioned in your first point. But now I disabled it.

Your second idea helped my to identify the problem better.

When I right-click on the friendly NPC the first time (without having this NPC as my target before), my slave does not realize my master's target changed and wants to interact with my masters' PREVIOUS target. WTF? It seems latency is the problem here. When I right-click on the same NPC again, my slave finally gets it as his target.

So...how can I improve this behavior? Any Ideas?