What is the Best Way to Control Two independent Joints on One and the Same Axis?

What is the easiest way to handle controlling a Cartesian machine that has 2 gantries that share the same space, except for where they park themselves at the limits of their Y-axis travel? Two gantries are used with their own tools/spindle vs having one gantry where the tools/spindles are swapped.
Simplified video of two independent motors sharing the same axis:

These don't have a long enough travel to interfere with each other but in my scenario they do.

I would think 2 independent gantries would be 2 Axis and 4 joints
(and fairly odd G code)

Sure, 2 axis with 2 joints each as 2 separate moving gantries on the Y-axis with a X-axis for each gantry and a spindle/laser/nozzle etc mounted on a Z-axis for each gantry. The video is over simplified, but conveys the Y-axis independent movement.

Now picture an X and Z-axis mounted on each of the voice coil motors, and each motor being able to move into the workspace of the other motor vs only 1/2 way of the travel of the common linear bearing.

Here is a better video:


and another:


short on pixels, but a good demo: