Did someone test the rendering times of more than one CUDA Graphic compared to one ?

Take a look at my PPBM web pages.  Adding a second card does more work if needed but unless your GPU-Z shows that your card is working at its full load it would be a waste of money.  The plot below shows this old card

working at its max with a timeline that was specifically design to stress the GPU.  Adding a second card does more work if it is necessary as in this case.  

Actually I have used two GTX 1060 6GB SC cards at less money than the more expensive single card solution.

It depends in large part on the CPU and PC platform your current system uses. If, for example, you had only a dual-core CPU or a non-hyperthreadable quad-core CPU, the second GPU would be a big waste of money because the CPU could not keep up with the demands of the dual GPUs. A second GPU might improve the rendering times on a PC with a quad-core CPU with hyperthreading, but the improvement (most of the time) would be too small to justify the expense of the second GPU.