Since you probably do not have a Slack install as yet to read the HOWTOs locally, then you might visit http://www.tldp.org
and search on RAID. I only work with RAID-1 and 5. But I think RAID-0 is what you are interested in. That would allow say for a 2 GByte boot partition, and then a 78 GByte storage partition on your (2) 40 GByte.
I think there is also LVM, logical volume manager, available. I think it might also provide what you are looking for. It is has been awhile since I looked at it but I think it would append drives.
Both RAID and LVM currently have kernel support options.
Who knows perhaps after reading the info you might even consider a root RAID-1 system. I use root RAID-1 on most everything I have of any real importance. I do not have the funds to purchase a 70 GByte tape backup or anything like that. And how else can you backup such a massive amount. For instance my primary workstation runs (2) 120 GByte in RAID-1. It is not even reasonable to consider a complete backup.