Combining two physical drives into a logical one

Solution 1:

You can combine them into a virtual RAID array (software raid) in the Windows Disk Management application. However, (AFAIK!) you can't do that to a system drive, NOR do this if the drive is already in use.

If you have a RAID capable motherboard, you can also use RAID0 to 2x the performance and size, but you also double the chance of something goes wrong (bla bla.. well.. backup regularly).

So no, I don't think there is an easy way to do this.
You can use "NTFS Junctions" and "hard links", but that's not exactly what you want. (Basically that would make you have a folder on C: (for example) which in reality is on D:.)
http://en.wikipedia.org/wiki/NTFS_junction_point
http://en.wikipedia.org/wiki/Hard_link