The problem with that argument is that more often than not, the two drives are in a RAID 0 or JBOD configuration. So even if just one drive fails, you'll lose all of your data anyway, making it no more secure than a single TB drive. The only way you benefit from data loss is with a RAID 1 or RAID 5 drive, but to achieve 1TB in either of those configurations, you would need either two 1TB drives or four 500GB drives.
I've had hard drives last me years without a problem. The chances of drive failure are pretty low, so just go ahead and buy whatever drive you want.