I got an MathWorks response:
From what I understand, you would like to display multiple x-axes in a "uifigure" component created in App Designer. Please let me know if my understanding is incorrect.
Unfortunately, this is not possible with the "uiaxes" component.
However, one possible workaround for this issue is to use regular axes objects in an App Designer application instead of using the default UIAxes objects. The regular axes objects cannot be added to the App Designer application with the drag and drop interface in the "Design View", but they can be added to a UIFigure programatically.
To achieve this workflow, a startup function for the UIFigure could be added to the App Designer application. Within this startup function, two new axes objects could be created, with the parent of the axes set to be the UIFigure. I have included a snippet of code to demonstrate this workflow within the startup function.
app.ax1 = axes(app.UIFigure)
app.ax2 = axes(app.UIFigure)
% Do additional processing to add plots to the axes, change positioning, etc.
Once these axes objects are created, you can perform the necessary manipulations of setting the positions, adjusting the axes locations, changing the background colors, etc.