I think your performance may end up being lower than what you have right now. It might be a lot of work just to figure that out.
Have you considered just adding the slower SSD to your computer, no RAID or anything? Use it to install new applications, and maybe offload a few existing applications if you are running out of room on the OS drive. Your system will boot up the same, and you won't have to go through all the work of building the array, reinstalling your OS, and reinstalling all your programs.
You can put smaller programs or those with less demands regarding loading time on the older SSD for the best balance.