I am thinking of applying for an Australian PR (skilled independent visa subclass 189). Both I and my wife are software engineers. For this visa would both of us have to get our skills assessed by Australian Computer Society (ACS)? Or is it OK if only the primary applicant gets the skill assessment?
Under the General Skilled Migration Points Test under SkillSelect, Partner skill qualifications are worth 5 points. If you think these 5 points will make a difference in you pass mark, you could both get assessed. If the primary applicant qualifications (could be either you or your wife) are enough, you wouldn't need. I would personally get both assessed, just in case some other points criteria, such as IETLS score, end up not meeting expectations, in which case those 5 points would make a difference.
As mentioned above, technically it would only need to be the primary applicant that gets a skills assessment.
But just to provide an other angle for looking at this, if you think of it in terms of how many points you get back for the amount of money spent, submitting your partners skills for assessment costs around $500 AUD for just 5 extra points. So pretty poor return on investment.
If you can manage it, scoring highly in the IELTS represents better value for money. You can get a maximum of 20 points if you score 8 or above in each category. It costs around £160 GBP ($340 AUD) to sit the test, so in my opinion focussing your efforts on a great score in the IELTS gives you more bang for your buck.
That said, 60 points is the minimum for a 189 Visa and you could get selected faster if you have more points than that, so any extra points you can get are useful.