I already checked AN214852. Thanks for the hints, specially the "Overview of SECI".
So SECI is only usable between products of cypress and it's specification is not public, correct ?
That's one of my 2 original questions.
My other questions is, can i enable 3-Wire coexistence interface on the CY43438KUBG ?
Yes, SECI is only usable between Cypress products and its specification is not public.
Currently, 3-wire coexistence feature is under validation and is not tested for CY43438KUBG chip. Please check this link Coexistence Throughput Test which uses 2-wire SECI and see if it helps you.
If it's critical for you to implement the 3-wire coexistence feature, I request you to get in touch with the local FAE or Cypress sales team for more information.