8-disk volumes

I'd like to create a raid volume from 8 disks using softraid. If I create a single raid 4 or 5 volume, the space efficiency is high (7/8), but only a single disk failure can be tolerated. If I create a raid 1+0 volume, the space efficiency is low (1/2), but 4 disk failures can be tolerated. I'd like something between this. I can accomplish this creating a raid 4 or 5 volume for 4 disks at a time. This gives me space efficiency of 3/4, and 2 disk failures can be tolerated (though of course not any two of the eight, but that's ok), which would be fine as a level of redundancy for me. But this latter solution produces two separate volumes, across which I have to distribute (and manage) my data. I'd like to have a single volume from the 8 disks with something like this level of redundancy / space efficiency. How can I achieve this with softraid? Thanks.