If both the disks have data on them, you will need a third party app to merge. I use
Paragon Hard Disk manager, but its not free, but there is a demo you can try. Someone else may know a good freeware app for you?
If one has no data, you can delete its partition and
extend the other to make it bigger. Right click
computer and click
manage. On the left down the bottom is
disk management. You can do it all there.