The theory in Ext 2 is only a little higher than than the concepts you are expected to grasp in Ext 1, the difficulty comes from the questions you will be expected to answer in exam conditions. Rather than asking you directly to recall theory or applying basic theory to a problem solving question, Ext 2 questions frequently involve using all the maths you have learned so far, even if it is not included in the Extension maths courses.

For example, I've seen solutions to questions that involved the ingenious use of the sine rule which we didn't touch in class since year 10.

So in other words, if you are feeling good conceptually with Extension 1, Extension 2 won't be much harder to learn. However, you will need to practice practice practice! I made this mistake last year, learning all the theory in the books but I rarely did any practice questions. End result was that I dug my own grave and got a much shitter mark than I wanted.

In 4U maths, by the time you realise you haven't done enough practice questions it is too late.