Since the speed of the cache can affect perf while working I'd generally say put it on the fastest drive which has a good amount of free space. I'd opt for a non-OS drive first but if the OS drive is the only SSD with a lot of free space, I'd place it there. You may need to experiment... but I personally just put in on the SSD... on my laptop that's the one internal SSD where the OS is located. In your case, I'd choose one of the two SSDs. Since your project is on that one SSD you mention, the cache on the OS drive may not be a bad choice if it has a lot of free space and not overused by non-video editing work while you are editing. An OS isn't necessarily always using its drive heavily... it's just the place where there is usually a lot of temp files, and other folders that are often used... so it could easily get overused but that doesn't mean it's a bad choice... just one whose characteristics you should be cognizant of. One of the forum threads below discusses cache location... see the other post below and search within it for "cache" and you'll see mention of spreading things around to different drives if possible. If you have two SSDs with a lot of space, I'd look to use them.
I moved this to the Hardware Forum where this issue has been discussed many times.