@cgrosbeck Yes, the firmware supports CoreXY IDEX, and has for a long time, so not "doable just"! And yes, it can handle the logic, the computations are not very difficult, see the diagrams below.
Machine kinematics are set by M669; see https://docs.duet3d.com/en/User_manual/Reference/Gcodes#m669-set-kinematics-type-and-kinematics-parameters
There are presets for the common machine types. I think what you are describing is actually what we refer to as a CoreXYU. A CoreXYUV would have two completely independent CoreXY gantries, including the X carriage (like @MJLew suggests). M669 already has support, and a preset, for CoreXYU and CoreXYUV. M669 allows setting up of custom kinematics, too. User @deckingman built a machine that had three separate CoreXY gantries, dubbed a CoreXYUVAB.
There are two ways of doing a CoreXYU (or CoreXY IDEX); the first is as you describe, with 4 motors for the X, Y and U motion:
COREXYU
The second is a CoreXY with a Markforged U axis, and uses three motors, and less belt and other hardware:
CoreXYU MarkForged
Both are supported by RRF, using the M669 K5 CoreXYU kinematic. There isn't any great advantage using the first over the second, while the first adds to the complexity.
M669 also supports the dual Markforged kinematic @oliof suggests. I believe @RogerPodacter has built one of these, see https://forum.duet3d.com/post/303241
a5b1a429-dd1b-4a14-99d5-74963644948f-image.png
(From this thread: https://forum.duet3d.com/topic/21021/dual-markforged-kinematics)
Ian