RAID 0 On 21.5" iMac Late-2012 I am about to get my fusion drive upgraded to a 256gb SSD on my Late-2012 21.5" iMac that I got from work (don't worry I have a company upgrading it, and yes they know its a Late-2012 iMac).  When that process is completed, it is my understanding that I will have two partitions.  The 256gb SSD that was installed, and a 128gb SSD left over from the fusion drive.  Can I RAID0 these together in a relatively reliable fashion? I understand the pitfalls of RAID0, and how if one drive fails they both go, but that is what Time Machine is for.  Has any had any experience with setting up this type of RAID on an iMac?
Generally speaking I BELIEVE that RAID requires drives of equal size. In some cases when you get drives of unequal size the RAID uses the size of the smallest drive, so in your case you would end up with a 256GB (128 + half the 256) RAID-0 Drive.

HOWEVER I have not done such a thing on Apple hardware using disk utility, it MIGHT give you a RAID of 256 + 128 GB, but I kinda doubt it.

But in any case creation of the RAID erases the data on both drives so if you are OK with that, try setting it up and let us know the results.