If you want to give the AAMC $235 to take a practice test without even being able to see what your score is or review the test afterwards, then congratulations, you've found the most expensive way to practice for the MCAT possible! Taking the MCAT and voiding it is almost entirely pointless in this case since you won't even have the benefit of finding out how well you did.
Also bear in mind that you have to wait until the second day following the exam to register for your next exam (you can only register for one exam at a time). Regular registration closes 2 weeks before each exam, so you'll have to do late registration for your second exam, which will cost you more money (about $60 if I recall correctly). Depending on where you live, there's also a good chance the second exam will already be full by then (in many places, seats fill up months before the exam date).
If you're truly concerned about being unprepared, I'd suggest buying some AAMC practice exams online (they're only $35 each), and paying $60 to reschedule your current date to a date when you will be prepared. What date are you currently scheduled for? If all you want is an extra 2 weeks and an extra practice exam, you can have this for a total of $95 instead of the $295 your plan would cost. With that money you could take almost all of the AAMC practice exams offered online instead. If you've already done all the AAMCs and you didn't do as well as you'd hoped, get the TBR practice exams and do those. Also look around at advice for retakers (these threads cover what to do when you've seemingly exhausted your practice supply).