RAID 0 On 21.5" iMac Late-2012
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.