What you want is called spanning or JBOD. Linux has had it since 1996, so Windows should get a good implementation around 2016. I'm not kidding there - Windows is consistently 18-21 years behind on enterprise features and five years ahead on their GUI.
For now, you can use a mount, where one drive appears as a folder inside drive C. The Unixes had that in 1970, Linux on day one in 1991, so Windows got it in 2003 - 22 years later. There is a small performance hit in the Microsoft implementation because they implemented it using drive sharing, the machine shares the drive to itself.
Right click on My Computer and select Manage. Then click Disk Management Select the volume you wish to mount inside of another and click Change Drive Letter and Paths From there you choose Mount in the following empty NTFS folder.
You COULD also use Windows immature spanning, but you won't be able to boot from either drive. See:
http://www.techfuels.com/storage/177...ed-volume.html