Bachelor of Philosophy, which at ANU is an extended research degree which is tailored towards your interests.
http://programsandcourses.anu.edu.au/program/APHSC
No you can't get in with an atar below 99 unless you have SEAS. Bonus points don't apply for courses with an atar above 98 so you'd need some sort of SEAS.
As for the two majors, you can do it if you do a double degree combining the phB and arts. Also you could use your electives to do an additional elective major (if there are enough points of electives available.
I'm fairly sure this isn't correct. SEAS (I'm pretty sure?) only applies for universities in Victoria and as ANU is in the ACT it doesn't apply, and the entry into the PhB, having such a small cohort, isn't as rigid as the entry into your average course, especially since entry is based on a range of criteria.
I got a place into the PhB for science this year and I deferred until 2016 (so hopefully I'd see you there if you decide to go for it and get a place!), so I've found out quite a bit about the program.
From what I've heard and seen, the 99 pretty much guarantees you entry, regardless of the rest of your application (ie. extra-curriculars and other similar things). These other things come into play when you get slightly below the score, so if you get a 98 or 97. As you're sending in a personalised application and two teacher references, the selectors are going to see if you've been disadvantaged, and I would say that this is something that they would take into account, especially if you've got strong references and you can prove that you're passionate about science/arts.
As for the two majors - the PhB definitely can't be combined to make a double degree. This is something that I wanted to do too but unfortunately it's not possible. You have to choose either science or asia-pacific studies and that's the area that you stick to for the majority of the degree, however you can do a certain amount of subjects outside of your main area - I've heard up to 6-8, however I'm not 100% about the exact number. Personally I plan to take up my offer in PhB science and then decide at the end of 1st year if I really want to study arts as well, and if I do, I'll try to transfer into a languages/science or asia-pacific studies/science double degree.
I'd just like to say as well - I didn't think I'd get the atar to get into the PhB and nearly didn't end up applying because of it, but I'm glad I did as I ended up getting above 99. All you can do is try your best in VCE and cross your fingers, and I hope it turns out well for you. But it's also worth remembering that you can transfer into the PhB from other degrees if you do well enough in your first year. There's always options!