It depends on how you want to use the drives.
If you want to go down the path of RAID you can either configure RAID 0 for performance gains at the cost of reliability or RAID 1 for reliability at the cost of capacity.
If you aren't interested in RAID then having two hard drives can be beneficial if you want to separate your operating system from your data. I.e. you have Windows installed on one drive and have all your saved games, music, videos etc on the other.
But if you are just looking from a capacity perspective then a single large drive is better.
Is there any reason why you'd be going for a 1TB drive instead of 2TB?