Hello Linda! Sorry for the late reply.
For application to Aerospace Engineering, there are both grade requirements as well as subject requirements. This means you must have taken the required subjects and scored at least a pass in them.
For NUS, you need a pass in H2 Maths and H2 physics/chemistry. If you did not take physics, you will need to have taken O level physics to take bridging modules.
For NTU, the criteria is similar, a H2 pass in Maths, and a H2 pass in physics/chemistry/biology/computing.
This qualifies you to APPLY for the DDP (Double Degree Programme). However, we all know that double degree places are limited and exclusive to only those the school deems to be able to cope. From my personal experience as a college consultant, you stand a better chance with Straight As or 4As. Beyond that, it would help if you have a decent CV with great CCA records or extraordinary achievements outside of school.
The essay you might need to submit will also play a VERY IMPORTANT role. If is normally 250-300 words and this is an excellent opportunity for you to paint the best story about yourself. You cannot change your grades or achievements but your story can package you well.
Once you pass the admissions stage, the next thing to do is to ACE your interview. You need to stand out from your peers and this can only be achieved through preparation and practice. If you need more help on the writing of the essay and interview, let me know and we can speak about those separately.
Good luck and all the best with your applications! Remember to input your backup choice. For eg, NUS allows you to indicate 2nd choice. Hit me up again if you need more info.