Caching does not generally improve performance. Once the cache is full, normal read/write speeds are what you deal with. The only time caching improves anything is if you are accessing a large number of small files. But in most cases, it really will not help and in fact will slow things down.
So what you do is you set up the 2 NVME drives in a mirror configuration as the “system” drive. The “system” drive is not the “OS” drive as the OS is spread through all the drives. But it is where the default storage are for your apps and you can use it for things like Virtual Machine hard drives, etc. Apps will launch more quickly off the SSD drives, etc.