Is Blender able to receive any benefit from using my 2060 RTX and my old 660 Ti since they both have Cuda cores. They're have different speeds and such but would they be able to take advantage when rendering in Cycles?
Cycles will be able to use them both simultaneously with the CUDA backend, each will render separate tiles. For several reasons though, the result will probably be slower than just using the 2060.
Since each card works on a different tile and there's no way for the other card to steal a tile once the first card claims it, you will eventually end up in a situation where all but one tile has finished rendering and the 660Ti is working on the last one while the 2060 sits idle. Because the performance difference between the cards is so great, this wasted time will likely be more than you'd save by having the 660Ti help in the first place.
Both cards need to have the scene copied into VRAM in its entirety. As a result, you're effectively limited to the VRAM of whichever card has less. Which is the 2GB 660Ti instead of the 6GB 2060
The 2060 has RTX hardware units, which Cycles can use via the OptiX backend to speed up ray intersections. However, the 660Ti is too old to use the OptiX backend. And the 660Ti is so slow in general, it is likely to contribute less than the RTX hardware on the 2060 could.
So while it's possible to render on both cards simultaneously, it is more likely to slow down your renders than just using the 2060 alone.
Try it and see what happens. It should work, but the older card will be significantly slower. Also, the amount of memory available for render will be that of the smaller card, so you might end up wasting resources from the newer GPU ,as it won't be used to the fullest.
But there might be some benefit of using two cards if you hook up the monitor(s) to the older card. That way you can disable that card for rendering and use the newer card. If the newer card is not using resources to display images in the screen then it can use 100% of its resources for rendering.