as patryn mentioned, the ultimate factor is cost. This is something that only your parents know. you have to find out how much your parents are willing to pay and choose the college that can give you the best chance and best yield. Once you find that out, you can PM me and maybe we can grab a coffee to discuss your options. (Much better compared to paying for USEIC centre in Singapore)
Alot of students wish to go overseas and i dont blame them. But have a talk with your parents first, after all school fees still have to be paid. Otherwise dont be too disheartened if you stay in SG. You can easily do an exchange or do postgrad overseas.
About double majoring... i recently did a bit of research as im interested in doing it as well. It is actually pretty uncommon to have a com sci and Geog major together. Reason is because it is very different; both in core curriculum and departments. Most schools offer a BS in Com Sci (altho there are many exceptions with schools offering BAs.) And Geography is usually considered as a BA.
More common double majors are Com Sci and Maths.
I agree with patryn as well... A major and Minor is better in your case. If it were math maybe a double major is fine but keep in mind of the GPA factor.
Finally, studying overseas isnt easy and you have to do a lot of research before going over. Covering the cost factor well so that your parents do not have to worry; showing independence in sorting out your own future and ensuring you are doing what you really want (in other words.. paying for something of real value) Unfortunately in Singapore we do not have huge resources or information about studying in US so it is really up to you to get as much as you can in terms of information.
I will be leaving in august. So let me know how else i can help you