I understand what you mean visually but technically the two planes cannot be “in the same position on the height axis” in the way you are describing.
The planes in this simulation are purely visual objects generated and positioned by the rendering code. They are not physical entities with a shared 3D position, collision box or independent physics state. Their position is calculated from their flight paths and movement parameters before they are rendered.
In other words, the game is not placing two actual objects at the same world-space height and then simulating them there. Each plane is being drawn at its own calculated position along its own trajectory.
What can happen, however, is a visual overlap caused by perspective. Because the scene is rendered through a 3D camera, two objects at different depths and/or different heights can project onto very similar or even identical screen-space coordinates from the player's viewing angle. This can make them appear to be at the same altitude even though their underlying 3D coordinates are different.
This is a standard perspective/occlusion effect: what you perceive on the 2D screen is the projection of the 3D positions, not a direct representation of their actual world-space Y coordinates.
So if the observation is that the planes LOOK like they are flying at the same height, that is absolutely possible as a visual illusion. But if the statement is that they actually occupy the exact same height coordinate in the simulation, that would not be consistent with how these planes are generated and positioned.
It's therefore more accurate to describe this as a visual perspective issue rather than the two planes actually sharing the same altitude.