An SSD is essentially already multiple planes per die in parallel * multiple dies / channels in parallel. It is essentially a RAID of its own. (Or RAIN if you prefer to call it that)
You can easily have 16 (channels * planes) in parallel on a mainstream drive.
Having two SSD in RAID essentially improves the speed by doubling the channels.
However you will find that random 4k to 16k is not going to have any difference, and the throughput increases with transfer size. Because at lower sizes, you are limited by latency. As the transfer size gets bigger, then the benefits of parallelism starts kicking in.
That's why you are only going to see speed improvements under situations where you can reach 3GB/s from a single drive in the first place.