At my school they don't actually let anyone start chem even at unit 2 without a holiday bridging class, and they definitly wouldn't let us pick up chem 3/4 (note they let people pick up basically every other subject except methods at 3/4 too so its not a blanket rule). There is A LOT of content which crosses over, and even if it isn't on the 3/4 course, the vast majority of the 1/2 content is build upon and is required as background concepts. I love chemistry, it's my favourite subject but i would not cope with picking it up 3/4 especially with the other subjects.
To be honest, maybe you could drop bio unit 2 and then pick it back up 3/4, not much content from unit 2 in bio is in 3/4 at all so if the blocking works that would be a good idea.
Also it would maybe work with spesh as i'm picking that up now, as all the 1/2 content is explained from scratch in the 3/4 course.
Hope that helps
