I did that last year cause I left the august PCAT thinking I had blown it (mainly since I had put off studying until the day before the exam). When I got my scores I found out I actually did pretty decently, but at the time it was too late to cancel my October PCAT. I took the October PCAT thinking I would do better since I actually studied this time but I ended up with the same score.
Basically to answer your question, take the August PCAT see how you feel, if you think you can improve in two months then sign up for the October one. But from my experience, you either know the material or you don't. Two months isn't enough time to drastically improve your score.
Take the practice exams and see how prepared you are. If you aren't ready, then take the October exam. It usually takes about five weeks to get your scores. Take that info and look at the regular and late deadlines, and the cancellation fees and you can figure out if you want to try for August too.