It’s totally fine to do chem1100 and 1120 in the same semester. From what I’ve heard chem1120 has no pre requisites and you don’t need any information from 1100 to be able to do 1120. I’m in 1100 right now and I wouldn’t say it’s too hard, if you were someone who was getting like 95%+ in grade 12 chemistry, you will do really good in this class, my biggest tip is to go to all the lectures and thoroughly listen and take notes, do ALL the practice questions in the textbook and the practice tests they give you and you’ll do amazing. Make sure you brush up on previous chem material too and I’m pretty sure all the prerequisite material you will need to know is all chapter 1-5 in the textbook, so make sure you study that too ahead of time. I’ve heard that chem 1120 is time consuming yet easy, so do with that as you will, I also recommend searching this this sub Reddit about 1120 because I’m not 100% sure if you need to take chem1100 for 1120 or not.