Make two disks appear as one big share on LAN

Finally settled for this option:

Pooling only software option: http://romanrm.ru/en/mhddfs

It allows you to have two disks with separate (and complete) filesystems, and make those appear as a single tree from the LAN. Directories with the same name will be merged when listed. You start with one drive and when its full, it will create the file on the other drive (if it has free space). If you disconnect a drive, it stops showing it's content.

Other options where:

Alternatives to LVM and JBOD (check "OS/software"):

http://blog.superuser.com/2011/09/14/building-a-nas-server-2/

EDIT: Other software option: http://snapraid.sourceforge.net/

JBOD and LVM volumes fail completely in case of a disk failure:

http://forum.elitebastards.com/viewtopic.php?f=6&t=16838

https://serverfault.com/questions/338517/what-happens-when-a-disk-fails-in-lvm

EDIT2:

As suggested by @lelouch, mergerfs sounds like a better mantained and updated option to mhddfs. On defense of mhddfs, I have been using it with no mayor issues for the last 4 years (apart that one or two times where it dropped the mountpoint for no reason, but after rebooting everything was in place again).

If not the slightest amount of reliability is required, RAID0 or JBOD get the job done.

Please be advised to not pick one of those if reliability is a concern. As you already found out, one drive failing would suffice to kill your entire array, doubling the chance of hardware failure when compared to a single drive.

Instead, you should look into RAID1 or RAID5 instead and buy one or two additional HDDs. Be warned though, RAID5 becomes more error-prone with harddrives as big as the ones you're using.