1) Buy an extra drive, internal or external.
2) If you don't have any, download backup software
3) Backup your OS drive. Test your backup. If you did a disk image, try booting from the backup.
4) Once you are sure that the backup is good, take out the current drives, build a RAID from the new drives, and restore your backup.
Now, some unasked-for advice. RAID0 is dangerous; you should always have tested backups. DO NOT put SSDs in any level of RAID. They will not receive the TRIM command and will slow down significantly over time.
If you need 480 GB of space, either buy a 480 GB SSD or have the two SSDs separate. Install the OS to one; install large games or applications to the other.
Why do they have to be in the same ports?