It sounds like all you care about is being able to utilize all 2TB of storage without having to manually place files on one drive or another. Either LVM or RAID0 can solve this problem for you at the expense of increased risk of failure. For LVM, you would make each 1TB drive an LVM physical volume and put them both in a single volume group. After that you could create logical volumes that up to 2TB in size. For RAID0, you'd just create the RAID device.
# pretending your unused 1TB disks are sdy and sdz
# for LVM
pvcreate /dev/sdy /dev/sdz
vgcreate myvg /dev/sdy /dev/sdz
lvcreate --name mylv --size 100%
# for RAID0
mdadm --create /dev/md0 --raid-devices 2 --level 0 /dev/sdy /dev/sdz
I don't know of a way to transparently merge separate filesystems into a single storage pool. This sort of sharding isn't uncommon, it's just typically implemented at the application rather than the storage layer. Engineyard has a paper describing filesystem sharding tactics and processes.