From what I have read on these forums, the consensus is not to RAID solid-state drives. 120 GB is plenty for the OS and all kinds of software.
If I personally had two SSDs, I would use the second for
1) My most frequently accessed and modified documents.
2) Scratch file space for all applications that use large scratch files.
3) Active projects for high IO work, such as video editing.
Let the drive suffer wear; I would get faster performance from my heaviest apps. I only have the one SSD, so I only do point number 1.