I am not sure of what you are attempting to do. Are you planning on having two MBWE II's each set on RAID 1, and mirroring each other, so that you have 4 times duplication? Or are you going to set up the MBWE II's on RAID 0, and then have the two boxes mirror one another for 2 times duplicity?
How much storage capacity do you want? the largest MBWE II's have 2T in RAID 1 mode (4T in RAID 0). If you don't want to have a MBWE II, you could instead get a MBWE (single drive) + MBPassport (or other USB drive). Connect the MBWE to the USB hardrive and use the Features Packs MB Backup hack from TeinturMan to do automatic scheduled backups from the MBWE to the USB hardrive.The largest MBWE's are 2T.
Alternatively you could by a WD Sharespace with RAID4 functionality.