update: cloned both disks and inserted in FreeNAS box. tried "zdb -u -l /dev/ada0" and got some uberblocks listed.
I could get back to a state with the -T option of zpool import but only to states in the past and AFTER the destroyed dataset :-(
Next thing - try to find uberblocks on the disk which are not listed via zdb and try to get back to this state.