Like suspected both devices are in the same pool, easiest way to fix is by doing the below, since I don't know which Unraid version you're using make sure you don't change the number of cache slots when removing the pool device, there's a bug in some releases.
-stop the array
-unassign both pool devices
-start array
-stop array
-assign both devices to the same pool, to the one where you want the existing data, there can't be a "all data will be delete" warning in front of any member
-start array
-stop array
-unassign the device you want to remove from that pool
-start array
-wait for the btrfs balance to finish, when it does stop the array
-you can now assign the removed device to the other pool
-start array, there will be an option to format that pool and once that's done you can use it.