Ross, If you have an Experimental Certificate and a VH- number - which you do - and you don't have a backup ASI and ALT - which you don't - then either the AP made a mistake in issuing the certificate (unlikely) or we don't need a backup ASI and Alt. I haven't seen that in any CASA EAB aircraft documents so far. Having said that though, I am putting both on my panel - personal choice based on my comfort level.