I'd lean towards exam 1s (without neglecting exam 2s of course) for a few reasons:
Even though exam 2 is worth double, there are twice as many marks there, so each individual mark in both exams is worth about the same. And if you go further and think about the fact that the cut-off marks for exam 1s are generally somewhat higher than exam 2 cut-offs, individual exam 1 marks seem more precious.
(e.g.: at least 80% on exam 2 (16 marks from the maximum) last year got you an A+, but you needed at least 85% (6 marks from the maximum) on exam 1 to get an A+ there.)
Another thing is, exam 1 skills can help with exam 2 -- namely, they will get you into the habit of doing some things by hand if you can see that it will make it easier. Sometimes chucking a lengthy equation into a calculator can be avoided if you do a little simplification first. But the converse doesn't hold: exam 2-specific skills (use of technology) won't help you with exam 1.
Note: I'm not saying that exam 2 isn't as important -- of course it is -- but try to squeeze in a few more exam 1s, because they don't take as long. If you do two exam 1s for every exam 2 you'll spend about the same amount of time on each, which could be a suitable arrangement. You want to eliminate the event of losing exam 1 marks on silly things, and if/when you reach that point, start to focus more on exam 2s.
The thing about exam 2s is you shouldn't jump straight to your calculator; only use it when you have to -- when it's a certain shortcut, or to just quickly check an answer. Look for ways to simplify things (by hand!) before you use it. This definitely helped me after I performed poorly when doing the 2010 VCAA exam 2. I went back and tried to do a few things by hand instead of doing everything with my calculator and it felt MUCH easier. They say students should use technology efficiently -- that doesn't mean do the entire exam with it, it means use it to save time without replacing your own brain. The real challenge in exam 2 is the actual analysis, working, and problem-solving -- which you are forced to do in exam 1, and hence those skills are the more universal ones.
In the end I'm hesitant to say outright that one should do more exam 1s, but hopefully I've helped with the decision a little.