I think you are right, this may only work when setting up the raid with 2 new drives. so, perhaps wont be able to make this work now. you should be able to undo the raid1 array, and have it retain all data on the first drive. check the documentation on that.
you could also just remove the second drive, and the raid tool will say you have a broken array. again, you should be able to tell it to stop the mirroring, and continue on with the drive intact. this may cause problems for getting any data off the second drive, if you have any there.
I would think you should be OK to use the 2 drives as separate, and if you are really concerned about data loss, do a weekly backup of critical data to the second drive. this will also give you more storage overall.
sorry I dont know a definite answer here, I havent used the SATA raids personally yet.