You need to connect them to a RAID controller or motherboard and configure them to be in the same RAID configuration as they were before. Seperately they have no data, togeather they have all the data that was saved. So, I'm not surprised you can't read the drives using a USB/SATA reader.
Imagine a file is a piece of paper. The computer needs to read that file to see that it's there and recover it. Half of that peice of paper is on one drive and the other half is on the other hard drive. So, like I said, seperately there are no files but togeather they have all the files needed.
(This also depends on the RAID configuration that was used.)