3.2 also includes more reporting on bus speed, hiccups etc.
If you send M122 (and M122 Bn for any CAN connected expansion boards with drivers involved in the move) you will see "hiccups" and/ or "underruns" start to be reported for very short segments with a high step rate. you can use those to determine when your step rate is too high/segment length is too short.
Note that for circles I have seen other printing artefacts become apparent before I hit underruns using short segments on circles but of course YMMV.
If you are saying that you want to print two different objects at the same time, then that will require very special preparation of the GCode. Your best option would be to generate GCode that has all the segmentation and Cartesian-to-Polar coordinate transformation already done, then you can configure the printer as a straight 4-axis Cartesian printer (XYZU).
If you don't want to do that but you do want to be able to switch between 2 tools, the simplest approach would be to have both tools run on the same linear rail and cover the same radius values. Then the coordinate transformation would be the same for both heads. You could use the existing Polar kinematics, and use the M584 command to switch the X axis to either the X or the U motor in the tool change files.
David, that helps a ton. At this time, I think the best approach for us is to prepare our own GCode and treat it as a 4-axis Cartesian printer.