Hi fi!
How strange! You definitely don't need to do English Units 1&2 in order to do Lit 3&4. I think it might be just thing at your school?
I can only assume that your school wants you to do English 1&2, just in case you decide you not to Lit 3&4 (and do English 3&4). The 2 subjects are quite different - you definitely don't need one subject to do the other, but it might be difficult to change if say, you did Lit 1&2 and then wanted to do English 3&4. So maybe your school is covering all bases and just making sure you can easily do English 3&4 if you need to?