I have root on sd0 and swap on sd0 and sd1.  If you can compile a kernel,
change the lines:

  config                netbsd  swap generic
  options               GENERIC

to something like:

  config                netbsd root on sd0a swap on sd0b and sd1b
  #options              GENERIC

I never tried it, but maybe this is what you want:

  config                netbsd root on sd0a swap on sd1b
  #options              GENERIC

--
John D. Smerdon;  Livonia, Michigan, USA;  Contents are my opinion.
Home: jds@smerdon.livonia.mi.us