How to use the engineering intercom channel?

I have 2 ATEM talkback converter 4K and ATEM 2 M/E Production Studio 4K. Prod. intercom works fine over SDI. I still need to use the intercom engineering channel. I know the engineering channel is SDI audio channel 13-14 and the prod channel is SDI audio channel 15-16. By default channel 15-16 is used, here is how do I use channel 13-14. Maybe there is an instruction somewhere, maybe I overlooked, maybe I need to check a box? I have searched the entire Internet answer to this question nil.