Question regarding K# for CoreXY to IDEX conversion

I have a CoreXY machine I'm converting to an IDEX by adding only a Cartesian Y drive and splitting the Carriage into two (by belt). The old X is now X , new Y motor added, the old Z, new U is the old Y motor replugged.
My matrix for M669 would be I believe
X 1: 1:0:0, Y 0:1:0:0, Z 0:0:1:0, U 0:1:0:1What K# do I use?
It's part Cartesian, modified CoreXYU?, or a double Markforged?I include this photo, not of my HW but of a system with similar kinematic requirements. My system has the pulley placement like a std Core XY.

@Strider007 Any help would be greatly appreciated.

I believe I need to ask DC42 for a new K? I would like to call it "CoreIDX".

The K value is used to select the kinematics family, and for those K values that select linearcombination kinematics, the default matrix. So if you define the matrix fully in the M669 command, it doesn't matter which K value you use as long as it is one of the Cartesian/Core family.

Great! To move forward I will use the IDEX config as a starting point as it already addresses the homing issues, and put in my own Matrix like I described above. Thank you.