You can use the LVM to create one large logical partition.
http://tldp.org/HOWTO/LVM-HOWTO/
physical volume: entire drives or partitions labled as LVM
Volume Group: Groups of physical volumes
Logical Volume: Sub partitions of a Volume group (seperate filesystems)
If your case you'd create one big Volume group with both drives it in, (leave out the /boot and / root partition). then create one big 20+ gig logical volume and filesystem for your file.