"Its not like you can 'SLI' 2 identical raidcards to get twice the throughput for 1 array."
Indeed, I know what you mean - near impossible to do with multiple parity RAID cards.
But - actually, I did once upon have a pair of IDE RAID cards that allowed you to do just that. They were of the fake/BIOS RAID type, and I cannot remember off the top of my head if they were based on the Promise or HPT chipset, but the BIOS on the first card detected and initialized all similar cards in the system and presented you with the disks from all the cards to use in one big RAID array. It could do this precisely because it was fake/BIOS/software RAID.