I'm not sure I understand you correctly, but I think you're saying that you have more space on the second drive and you would like to lend that space to the first drive?
If so, you can set a folder on C drive to redirect any requests to a location on D drive. for example:
C:\Games points to D:\Whatever\Games
Any programs requesting information from say C:\Games\Quake will think that's where they're looking but are actually looking in D:\Whatever\Games\Quake.
I used to use a sysinternals command line program called 'junction', and the command was:
JUNCTION -S C:\Games D:\Whatever\Games
There are newer techniques as Windows introduced features like symlinking but I'm a creature of habit and since NTFS junctions have almost invariably worked for me, I stick with them. Ah yes, that's a point, this folder redirection feature needs a NTFS filesystem on the source location at the very least.