Boot into recovery, go in cmd and type adb.
type
dd if=/dev/block/bootdevice/by-name/boot_a of =/data/media/0/nameoffile.img
dd if=/dev/block/bootdevice/by-name/boot_b of =/data/media/0/nameoffile.img
patch one of them in magisk manager, fastboot boot patched img, direct install magisk in...