Hi, I just finished up Global 3/4 this year without doing 1+2 and, to be completely honest, I've never needed to go and specifically look into 1+2 Global. It's definitely a subject which is eminently doable without knowing what's covered in 1+2.
In terms of preparation, my first and foremost suggestion would be to begin to know the study design inside and out. Even though Global is an incredibly wide-ranging subject, the study design places limitations on what can (and will) be asked and hence will help you direct what you need to know.
Other than that, reading up on international relations news and being aware of what's going on will help immensely. Good resources to start with are Foreign Policy magazine, The Guardian and The Economist. Begin noting down any events you believe are pertinent to the study design although admittedly, some of this will be difficult as many specific examples will be dependent on which focus areas your school decides to do within each area of study.
Hope this helps and good luck!