3 ssd in iMac problem

#1

hi i wonder if anyone can help? i have a late 2015 27 inch 5k iMac, i7 4ghz, 32 gb ram, it was originally 3tb fusion drive with the apple 128gb blade ssd, i have since replaced the blade ssd with a 1tb version (yes had the whole mac to bits) i then decided to upgrade the 3tb rotational hd with a samsung evo 850 4tb ssd, this was placed in a startech 2.5 to 3.5 inch DUAL enclosure, (room for another drive ), this then worked ok and fast boot , I'm on high sierra i should add and not using apfs system , i then installed a SECOND samsung evo 850 4tb ssd in the second slot of the caddy and this is where the fun began!! i could only get disk utility to see one ssd and no matter what i tried the only way i could utilise the 2 ssd (using the raid selecter switch on the startech enclosure and not in disk utility) was to USE RAID 0, then disk utility could see the drive as AXMT 109X RAID 0 8TB so i partitioned this drive Macintosh HD and os extended journaled and all seemed ok, boots in about 11 seconds, BUT--------- disk utility and system info reporting the disk as non ssd rotational, i have a sneaking suspicion its to do with something called port multiplication which i don't think iMac supports? what i really would like to know is am i ok using the two ssd in this manner or will they be damaged? i enabled trim in terminal in case it helps, also stats reporting the drive as 46c as of typing this. any help would be gratefully received, many thanks.