The 1660 has 1536 CUDA cores, the 2060 has 1920 CUDA cores. But if you have both cards in the PC you have 3456 CUDA cores to render with.
So if possible you can just put them both in your PC and use them both to render, no NVLink needed to just render with multiple GPU’s. I had 2x1070 and later 1x1070 and 1x2080Ti in my PC. The amount of available memory will just be the memory on the card with least memory. In your case both have 6GB if I’m right so that will stay 6GB.
To get back at your last reply, besides things of the clock speeds of your GPU’s the more CUDA cores the faster the rendering process will go.
I’ve now a 3090 in my PC and a 2080Ti that I want IN my PC but it doesn’t fit
the 3090 has around 10000 CUDA cores but the 4000-something of the 2080TI would be welcome as well. That will reduce the memory I can use if both enabled since the 2080Ti has 11GB and the 3090 has 24GB.
If you have the space in your case I would put them in both but it can be hard to cool if they are real close to another. Although 2x 1070 never gave me a problem being really close together but the newer cards get really hot.
And if you want to do a good render test, take a bit of a complicated scene or let it stop with quite a lot of samples. Transferring the textures and geometry to GPU also costs time and if you’ve really short render times they have more impact that if you render for a longer time. That’s also the reason why I’m no fan of the benchmark of KeyShot. I can overclock my card and get a high score in that benchmark but if I actually us it that way in KeyShot the drivers of the graphics card instantly crash 