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