I always played it like this:
Rat Activates, immediately ends its Activation, another one of my models Activates.
My opponent's Activation.
Rat Activates...
Core Rulebook, Page 57, Chain Activation reads:
Players ma not Activate more than two models in a row this way, unless they are Activated by the same effect.
I thought that the same effect means... How to explain it 😕
The very same Action, not just an Action with the same name on a different model.