Using 1 SSD to cache the new one? Absolutely no reason or benefit to do that.
Using it to cache the storage drives? Don't do that either.
1. You can only use 64GB of it for the cache
2. The cache works by learning the most used files, and those end up on the SSD. Since you almost certainly aren't accessing the same files from storage all the time, it still has to come of the HDD.
Just have it as another drive. Maybe install a game or two on it. Or use it for whatever.