Why can I not combine two partitions into one from Disk Management?
Move all of the files from one of your data partitions to the other. In disk management, delete the now-empty partition. You should then be able to grow the remaining partition to fill the empty space.
If that doesn't work, move all the data files off the system (onto an external USB drive, for instance), delete BOTH data partitions, and create a new partition that fills the now-empty space.