Go to "computer management" by right clicking the computer icon on your desktop and selecting "manage".
Choose disk management, right click on the 2tb drive and select "shrink volume". Shrink it to 1tb and after formatting and restarting you will see a 1 tb partition and 1 tb unallocated space. On the unallocated space, right click again, create a "new logical volume" and allocate the rest 1tb volume to the new volume and you will have 2 X 1tb partition.
By the way, I hope it is not the disk containing the OS, otherwise, you will have to format the disk and partition while reinstalling the OS. Also remember to backup all the data, as partitioning will format your disk and erase all the data.
If you have any problem, use this guide...