I believe you can double major if you're doing a single degree.
For now, you just follow the stuff in the handbooks. So just enrol in level 1 courses for the majors you want to complete - up to 4 of them. Don't enrol in more than 24uoc of courses per semester. 24uoc is normal load.
For UNSW, remember you have to get a bus ticket as well. Get MyMulti tickets. I get them weekly.
If you're more responsible with your belongings, try a monthly or something else. But remember to be careful to figure out when semester breaks occur.
tl;dr For UNSW, MyMulti tickets. ALWAYS.
You pick subjects so you have 4 for semester 1.
Pick any stage one course for Pharmacology.
Pick any compulsory Commerce core, or flexible core (you have to do two of those eventually)
And keep picking courses from the above two categories until you have 4.
Commerce has core courses you can read about in the Handbook: http://www.handbook.unsw.edu.au/undergraduate/programs/2012/3502.html
For Pharmacology major, you look at the Handbook entry for that major: http://www.handbook.unsw.edu.au/undergraduate/plans/2012/PHARA13529.html
So it's all in the...
Well, if you want to do Quant Risk and are accepted into it, then of course.
But you get invited into that program, so go up one day and ask if you're invited into it. If not, then I think there's no use for you to be doing 51 in a Maths degree.
No, MATH1151 is aimed at Actuarial students. MATH1141 is for Mathematics students. Course content is similar, but different in bits. If you want to be an actuary, do 51, if you want to do maths, take 41.
I think you can enrol up until the first day of semester. But you'd get crappy times. Get in now while there are vacancies in classes so you can pick and choose what classes you want.
Oh I almost forgot, the School of Maths website has plans for the subject. Plans that outline what courses you should take for different majors for each semester.
http://www.maths.unsw.edu.au/futurestudents/commerce-advanced-maths
Scroll down.
There are different lecture streams throughout the week, that cover the same material.
Screenshot it and post it here, because I'm still not sure what's going on.