Intel do it and have done it for years, it's called matrix raid.
Eg. you put in two drives, split the drive into two partitions, 1st half is in raid 0 with the other drive and the second partition is in raid 1 with the other drive for redundancy.
I have only used it a few times, even then...
Your write speeds will be abysmal on the software RAID 5 array, no XOR engine means that the cpu has to calculate parity for the array too. (hogging lots of cpu cycles). My experience is that software RAID also takes ages to initialise and rebuild compared to a proper RAID card.
I used to...