You have to get 80+ in 2u and 78+ in chem (to get a 90 ATAR), assuming that you get a band 6 or (or at least something close to that (around 87) in your other subjects)
I suggest forgetting about your internal marks for now and work on performing in your trials and hsc, reason for that is the internal marks are highly dependent on your rank, cohort performance and the gap between you and the people at the top. However, if you perform really well in your in your hsc, you might be able to neutralise or at least reduce the effect of your internal mark on your final hsc mark. For now, I highly advise that you study hard for the trials and try to save what can be saved in terms of your ranking for 2u and chem
Improving your marks 2u is fairly simple. What you have to do is firstly do a past paper or two to figure out your major weaknesses, and then grab a textbook thats not too difficult (Coachman & Jones or Maths in focus) and work through the exercises that cover your weaknesses. Another option would be to get a good tutor to help you close your content gaps. Another thing you could do while fixing your content is to do drills, so for example pick up some old 2u trial/ hsc papers (1995-2005) and do the first 5 questions (Q1-Q5) and keep doing that from different papers until you get full marks, then move on to q6,7 and 8 and do the same thing. By the time you're done with fixing your content and doing textbook exercises you'll also be familiar with the structure and questions of the 2u paper. Btw make sure you check the worked solutions and go through your mistakes, and re-attempt the questions you didn't get right.
When your content is fixed or close to being fixed, start doing past papers, practise as many as you can and under exam conditions, then mark your paper and go through your mistakes (THOROUGHLY).
For chem, find yourself a good set of notes and go through it, and then do questions and past papers.
It's very very important that you do not neglect your other subjects, you need to maintain your ranks and marks while fixing your weaker subjects.
It's possible to get a decent mark in both 2u and chem, you just have to work hard in an efficient and productive way. All the best!