Defraggler is a good free program. It is from the same company that makes CCleaner.
If they are two physical drives where drive A has the C:\ drive and the drive B has the D:\ drive there is no way you can extend the D:\ partition with space from the C:\ partition. Being two separate drives there is no way to share the space among them.
The Unallocated space means that the newly created partition needs to be formatted for windows to recognize it. So if it is still with the first drive you can add it back to the C:\ drive or you can make the space created to its own drive.
If they are two physical drives where drive A has the C:\ drive and the drive B has the D:\ drive there is no way you can extend the D:\ partition with space from the C:\ partition. Being two separate drives there is no way to share the space among them.
The Unallocated space means that the newly created partition needs to be formatted for windows to recognize it. So if it is still with the first drive you can add it back to the C:\ drive or you can make the space created to its own drive.