URD/Thorly pipe bomb Y pipe question.

Just picked up a slightly used one.
I understand that it replaces the rear cats with resonators.
If I follow it correctly I need to use the O2 sim unit to "fool" the computer correct?

Now since the rear O2 sensors are up stream of the rear cat/soon to be resonators, how do it know if the rear cats are there?
It is picking up the O2 out of the first cats and there is no sensor after the second cats.

Help me out here, also since I am keeping the front cats do I need the O2 sim unit?

Thanks up front.