There are many examples of scaling gizmos in 3D software with perpendicular scaling handles.
Observe the colored squares on this gizmo
They are colored to the axis perpendicular to the handle. The XZ handle, for example, is green. Dragging this handle will scale along two axes at once. Importantly, this scaling is uniform between the axes. Each one is scaled at the same rate based on the mouse’s distance from the center of the gizmo.
Roblox should add these handles to the scaling tool.
Here’s a video of perpendicular scaling handles in action