I've confirmed with NetBSD/9.0 amd64

• prepare two scratch (the same) disk
• at sysinst, those two can be formatted as raid0
• Can be installed to raid0
• But manual install boot is necessary to boot from them as described at: 2016/12/26

  # mount /dev/raid1a /mnt  (mount just installed root)
  # chroot /mnt             (getting installed filesystem environment)
  # mount -a                (getting all installed filesystems)
  # df -m
  

  # cd /usr/mdec
  # installboot -fv -t raid               /dev/wd0e bootxx_ffsv2 /boot
  # installboot -fv -t raid -o timeout=30 <font color="red">/dev/wd1e</font> bootxx_ffsv2 /boot
  
  

See detail herel (ps axd output)

CF-SX3@makoto 00:29:54/200329(~)% cat prep/cargo2                             
 6930 ?     Is     0:00.08 | |-- sshd: root@notty 
 8735 ?     Is     0:00.32 | | `-- /usr/pkg_bulk/bin/pbulk-build -c 192.168.60
15383 ?     I      0:00.03 | |   `-- /bin/sh /usr/pkg_bulk/libexec/pbulk/pkg-b
 2213 ?     I      0:00.87 | |     `-- /usr/bin/make all BATCH DEPENDS_TARGET 
 4652 ?     I      0:00.00 | |       `-- /bin/sh -c set -e;\t\t\t\t\t if test 
 3883 ?     I      0:01.04 | |         `-- /usr/bin/make _MAKE OPSYS OS_VERSIO
12764 ?     I      0:00.05 | |           `-- /usr/pkg/bin/gmake -j4 -f Makefil
 7039 ?     I      0:00.04 | |             `-- /usr/pkg/bin/gmake compile 
29679 ?     I      0:00.34 | |               `-- /usr/pkg/bin/gmake recurse_co
21591 ?     I      0:00.02 | |                 `-- /usr/pkg/bin/gmake -C toolk
29688 ?     Il     0:45.37 | |                   `-- /usr/pkg/bin/cargo rustc 
23504 ?     I      0:00.00 | |                     `-- /usr/pkg/bin/cargo rust
22090 ?     Ss     0:00.08 | `-- sshd: root@pts/0 (sshd)