Two seperate G-Speed 24TB hard drives (48TB total)-only giving me 18TB after RAID 1 setup on Mac disc utility

I have a total of (two) G-Technology 24TB G-SPEED Studio Thunderbolt 2 external hard drives (new, but 5 years old). The two separate hard drives have a total capacity of 48TB.
I used my iMac disc utility Raid configuration for RAID 1. After completing the setup, I’m only showing 18TB now mirrored. Shouldn’t it be 24TB? Also, I do mostly Video editing and I set the stripe size to 128k.
Should I have chosen 256k?
Thank you for any response.