The "easiest" setup would be with a multi-attachable disk shelf if you have some laying around and only want two-node ZFS for e.g. a SAN (iSCSI or FC). You then "only" need the HA and ZFS part. I have this setup still on my todo list for try out.
Technically it will be possible what you want, but it'll be very slow. There is also the general rule not to use a COW on a COW system. I read this everywhere, yet I never read an explanation (maybe someone can jump in, IMHO is just speed)