There is an easier way to get NetBSD/OpenBSD dual boot.
Install NetBSD and at this
step allocate MBR partition a: to NetBSD. Add a Bootmenu entry signifying that it is "NetBSD". The use one of the remaining MBR partitions for OpenBSD. Be sure to specify the partition type as OpenBSD (A6) and add an "OpenBSD" Bootmenu entry.
After you complete the NetBSD installation, install OpenBSD to the OpenBSD partition.
If all went well, on boot select 1 for NetBSD or 2 to boot OpenBSD