In EC2 the i2.2xlarge (or i2.4xlarge) instance type come with 2x800 GB SSD (or 4x800 GB SSD) for instance storage.
Is there a way (using RAID?) to make these separate disks be treated as a single larger 1600 GB (or 3200 GB) disk? How? And are there any important reason not to do this?
Zoredache answered it quite well there. Answering the last question: You will be increasing the probability of failure in a big way since the failure of just one of the devices will make the whole virtual disk fail. That's why most of us use RAID 10.