Your screenshot shows just a single hard drive (sda), about 465 GB, with three partitions, sda1 is /boot/efi, sda2 is your main Linux installation mounted as /, and sda3 is your swap area. You can see that there are not TWO partitions in the 400+ GB range, just the one where one of your Linux installs is located.
If you click on the down arrow in the top right box, does it show another drive, like sdb? If yes, that would likely be the remainder of the 1 TB space you have. But I would think that is a separate physical drive, not a single drive of 1 TB. And if that is the case, you can make two disks look like one, but it is probably more complicated than it is worth.
Cheers!