Hi. It sounds like they're just (very simply - not the expensive & difficult voltage or defective hardware problems) both configured to recognize as the primary harddrive on the system board, so the computer's BIOS can't see either one because they're both detecting as the primary.
Disconnect 1 drive then reboot & see if it detects correctly, then switch the drives & do the same for the other one.
If it sees both drives seprately they're both fine, but they need a jumper or other setting changed to work together. You should also look at where the motherboard cables are connected to the mainboard, You may have plugged the SATA drive into the primary socket, but the SSD defaults to the primary boot device, so it may sending that signal through the secondary data cable connection.