I still have a few questions:
1) Are you sure you have an "MKR" 350 and not an "AMR" 350? The MKR is not an audio panel or intercom at all, it's merely a marker beacon receiver and would not in any way work with the install you're attempting to do.
2) Assuming you have an AMR-350, you need to understand that these haven't been made for about 30 years - is several generations out of date, and again it's only an radio switching panel - NOT an intercom which you still need to buy. They were good in their day, but now - well.....
3) Do you have an intercom in addition to this audio panel? If so, all intercoms have separate pins for the pilot/co pilot mic & headset jacks.
I suppose this might not be what you want to hear, but considering that this is a new airplane with nice avionics in it, I wouldn't recommend even installing that dinosaur. You'd be WAY further ahead to just buy a good intercom or even entry level modern audio panel.
In the end I still can't give you the right answers because I don't exactly know what you have yet. I'd love to try and help you and get you some good information. I tell you what, why don't you send me your phone number off list or give me a call so we can get this straightened out for you.
Best Regards,
Stein