There are several ways to do this.
If your motherboard supports hardware RAID you can set up a RAID 0 array with the two, then reinstall Windows. Many people here will tell you that this is a bad idea due to increased risk of data loss if one disk dies, and it also takes a fair bit of effort. But for high performance PCs with everything backed up regularly this can be an option worth considering.
Option 2 is Storage Spaces as suggested by Darius. Setting up a combined storage space will wipe any data that was on the disks used, so not particularly useful with your boot drive.
Option 3 is to symbolically link the second disk into the first in a location that will gather lots of data. I'd suggest moving Program Files and Program Files (x86) to the second disk and sym linking them back to the C: drive so that it looks like they are where they were originally. Program data would install to the second disk, while Windows and your Documents would stay on the first disk, but everything would be accessible via C:\
You would still see the second drive as D: and could access the data on it via D: also.
This option should work without too many issues other than occasional minor confusion over the fact that you can get to the same files via 2 paths.