SSD for Mac Book Pro 2012 optical disc drive
SATA interfaces are backward compatible so you can use a SATA III drive on a SATA II/I port.

From the SanDisk Knowledgebase:


  SATA II specifications provide backward compatibility to function on
  SATA I ports. SATA III specifications provide backward compatibility
  to function on SATA I and SATA II ports. However, the maximum speed of
  the drive will be slower due to the lower speed limitations of the
  port.


I have actually done the upgrade you are describing (though with one SSD and a spinning drive in the optical bay).  There are no issues with SATA compatibility.