I'm not completely sure, but I don't think you'd get away with not doing any other arts courses than the languages, if you did B Comm/Arts. I don't know what ^they're thinking.
I do Advanced Science at UNSW. I was considering doing Science/Arts, but then I realised, all I wanted to do was learn German. So now, I do German as an elective. Out of the 192 units of credit I do over my 4 year course, I can do 42 units of anything - i.e. I'm not completely restricted to doing science subjects. Basically, lanague courses are 6 units per semester. They're split up into Introductory, Intermediate, Advanced and Professional for each year, then into A and B for first and second semester. If you're learning a language from scratch, then finishing the Advanced course for that subject too, you'd be looking at 36 units. (introductory A, B, intermediate A, B, advanced A, B.) I'm sure the requirements would be similar for a commerce degree.
So, you'd either need a course with a lot of flexibility to do two lanagues, from a complete beginner's level to advanced, OR if you've got previous experiences with these languages, you can start off at a higher level. I'm starting german this semester at Intermediate B because I did german continuers for my HSC. I'm not even sure I'll continue doing german for very long. I'm definitely not continuing german at uni past the end of 2012. I'll either just stick to science courses or pick up another language like Spanish or Italian or something.
In my opinion, I'd recommend you do comm (lib studies). But, you know, I do science, so what can I possibly know?
It also depends on how far you want to go learning these languages.
You should check out the program requirements online. So like, check how much of what kind of subjects you need to take and things like that. Not sure about other unis, but for UNSW, you can look this up on:
www.handbook.unsw.edu.au
Good luck!