Hi Lauren!
I did Literature 3/4 without 1/2. Someone who has done 1/2 would give you a better idea of what you might miss out on, but from my perspective, I would recommend doing 1/2 if you can.
The main thing to realise is that English and Lit are two very different subjects - different criteria, different writing style, different approaches etc. In that sense, doing Lit 1/2 would be a huge advantage. English is essentially a continuation of all the English you've done at school from year 7 to year 10, but Lit is incredibly different and having one extra year of practising 'literature-style' writing would definitely help you feel more comfortable in the subject.
That said, we were taught all the necessary things around formatting, close analysis, essay structures etc in 3/4 so you definitely won't be miles behind if you jumped into 3/4 without 1/2! However I'd still recommend doing 1/2 because you would be getting that extra time needed to refine your writing.
The most important thing is - you should check with your school to see if they allow students to do 3/4 Lit without 1/2. A lot of schools don't (unless under exceptional circumstances) so if you haven't already, definitely get that confirmed.
Hope this helps! Feel free to ask if you have any more questions!