Hi 2118kool,
Many GMATers use a variety of different study materials as they prepare for the GMAT, so there's nothing inherently "wrong" with using two separate sets of course books. That having been said, there are some broader issues to consider.
First, the immediate issue would likely involve the different attitude and emphasis of each product. Most GMAT questions can be approached in more than one way - and one book (or set of books) might place a greater emphasis on one type of approach. By extension, you might only practice one method - and potentially end up splitting time between two methods without ever really learning when to use one approach instead of another.
Second, unfortunately, many people who use a 'book heavy' study approach end up getting 'stuck' at a particular score level. Even the best books are limited in what they can teach you; they also can't force you to approach questions in a certain way and their explanations are often one-sided. This is meant to say that you would likely find it beneficial to invest in some new, non-book study materials at some point in your studies (and the earlier you make that choice, the more time you will have to hone your skills and the greater your chances of scoring at a higher level).
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied so far?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich