There are 3 parts to become a Fellow Actuary.
By obtaining the first 2 parts, you'll only be able to become an associate, and you'll not be able to sign as an actuary.
To obtain part1, you just need to be within a recognized university and undergo an actuarial degree with a satisfactory mark (above credit for subjects that provide exemptions).
To obtain part2, you'll need to take 2 or 3 more subjects (depending on your university) after fulfilling the 3 year degree and achieve a satisfactory mark (above distinction average in USNW, credit in Macq etc). So you'll need to do either an honor, double degree or co-op, to help you guarantee that 4th year.
Part 3 is done after your 3/4/5year course. There's 4 exams to take for part 3.
So essentially you need the double degree to get part2, unless you have other means.
As to combining with commerce, it really depends what you wanna do after uni.
You can combine it with commerce, economics, Adv. Science, and Law. Though, I don't see many people that combine Actuarial studies with law, but commerce and Adv. Science (mathematics) are very popular. I'm personally doing Actl/Comm, and it's a pretty good choice, as you get to choose 3 majors (including actuarial.s). Combining with commerce is a waste if you're not gonna achieve part2 (but then why would you be doing the course?).
Just as a side note, single actl degree provides you with 2majors to choose (fins,acct, etc).
