Can OS X Combine Physical Disks
Yes - OS X offers software RAID functionality on both the Server version and consumer OS. Open Disk Utility and read through the help on RAID.


  Create a single, large disk from several smaller disks by creating a concatenated disk set, which is also called “Just a Bunch of Disks” (JBOD) or “spanning.” The concatenated disk set acts as one large disk with the combined capacity of all the smaller disks.


You will need to erase/repartition both drives and then can choose from a simple concatenation of the drives or set up striping which has advantages and disadvantages compared to just combining the two drives.