Opacity because of polygon overlay not working correct?

Hi There

I want to switch from one object to another in an animation... so I wanted to make a trasition with opacity 1 to 0 on one object and from 0 to 1 on the other object within a short time. This was always the easiest way in Potoview.

To show the issue, I placed a red and jellow spere over each other (same size).

Now this happens in Visualize, when I set the opacity of the red sphere to 0.

A solution is to reduze the size of one sphere to 99%... then it works well. But I hope somebody knows an other solution.