No, of course not, how do you expect an SSD to cache another SSD? Have you ever asked if an HDD can cache another HDD? It's about the same question.
Microsoft's ReadyBoost can be used to turn an SSD into a caching controller, but it is limited to 32GB, and it only works with hard drives. Microsoft also is smart enough to know that it disables ReadyBoost when trying to cache an SSD.
You might as well either get the 250GB SSD + a really large HDD (3TB+), or just get the one 1TB SSD.
Generally not, as both drives will be comparable in speed, assuming they are both SATA drives. Also if you're talking about Intel's Smart Response software, it only allocates 64GB of a given SSD for use as a cache anyway.
it would probably be faster if you had 2 512 GB SSDs set to RAID 0.
faster than what?