Regarding Mixing and Matching.
In my personal studying: I went online, found the reviews from students - and ended up buying both. I read that Kaplan was good for thorough content (which I needed) and then EK had some practice problems (which were far and few in between back in 2015) - the year the new MCAT came out. They were both easy to hit the trigger on both and I ended up with ~$280 in books which was within what I thought was reasonable when it came to books for the MCAT. At that time there wasn't much of a used marketplace since the MCAT was so new, so I was forced to buy them new and I just was like, I'd rather be safe than sorry. I ended up using pretty much one set as that suited my learning style better and not much of the second. So certainly figure out if the learning style of one set works for you. DO YOUR RESEARCH and don't let books collect dust and/or waste your $ (there are many more expenses post MCAT to worry about).
How I personally studied for a 128/124 (EEK, it never got good)/129/126 (artificially low, i forgot to upkeep) was I read through Kaplan (I needed thorough) in entirety and did all the in-chapter questions and end-of-chapter questions. Tried to make a big picture framework of "WHY" and then after a few months of reading these a few hrs a day (slow reader, was learning some things for the first time), I decided to do some EK problems. Upon the 30-minute Physics exam (in a topic area i am EXCELLENT at) and in/end-of chapter MC questions - I was scoring so poorly and spending so much time I nearly gave up. I stopped using the EK books, although I had scheduled to do "the 30 minute exams" many times - it just didn't end up happening (although I'm sure if they don't demotivate you, they are great practice). I did use them for the following chapters: Ochem Lab techniques ,a second perspective on biochemistry amino acids, structures, and metabolism, along with the Reasoning book (needed another perspective on CARS, some MCAT math tops, etc) so I don't necessarily regret purchasing it considering my personal financial situation - but if you have limited dollars I would highly recommend starting with one. One set will always be cheaper than mixing and matching 2 fyi - but go with what works best for your learning ofc.
What I did use for a second perceptive more than EK and for practice problems - was the KA videos, the MC questions there, and the passages (during my content review phase of my learning). Less daunting for me personally which made it more enjoyable to study. Maybe if you are already a great problem solver, and like the challenge of the EK problems (and the concise summaries/colorful pictures - EK would serve you a better supplement to EK than it did for me). For me - the bulk of my studying was done with kaplan and KA and I followed this up with a few FLs + AAMC materials (where my reasoning was really refined and content gaps were exposed and filled in). I loved KA for Psych/Sociology and Biology/Biochemistry especially and in retrospect KA for Psychology/Sociology alone would have done the job. I forgot to refresh my knowledge on Psych though a month leading up to my test and noticed a drop on my real performance.
The whole idea of "if XYZ is fresh in your mind I can use a less thorough resource" is silly to me, even if you of an a average reading speed and you already know your stuff, you can get through even a very comprehensive set very quickly (few days). I can read an entire Kaplan book on a subject I already know in 12 hours. If I don't know it it would take me like 50. How much time are you really saving by having a "shorter" source is what I always ask students. Do you want to cut 5 hours? If time is of the essence, and you're very confident in your abilities and/or the conciseness actually suits you - I totally understand.
Regardless - if you're good in a subject - just go over it quickly in any way (any source is fine) and then do practice problems. EK has more than Kaplan which is good and will make them a better fit in your situation for those classes (Bio, Gen Chem Ochem). If you find that after some practice you don't actually know everything or upon reading EK you find oh shoot, I don't actually know about "ABC" - go read/watch a video that is more thorough than the EK books and fill in the details (this can be a video online, wikipedia, another book set, etc). EK CARS book is much more efficient than kaplan, especially since CARS is via practice (although I didn't do much of this, and my score is fairly low (and was such on practice as well) so take my advice with a grain of salt). Kaplan biochem is excellently detailed - and the exam is as well - if you do this, and then get a second source for the lab techniques just in case - you will do great. Physics in Kaplan was also great - but honestly didn't have that many practice problems - so I went to Khan academy did all their MC questions and watched some videos on things I struggled with and went to the EK books and did a few questions as well.
The MCAT without Physics and Biochem will mean you're gonna have to dedicate some time (beyond just reading any one set) and practice for it thoroughly. Berkeley Review seems to be a popular choice for those who have never taken X subject before and it seems to have so much practice that it leaves little room for error. As one of my friends who used Berkeley Review said on a Live Session on Instagram regarding his MCAT preparation, "if you use Berkeley Review and do all the problems, I do not see how you can get a low score" (he had only taken Physics during highschool and never in college before his MCAT).
TPR/KA for P/S seems to be the source to go too! I've seen great results from students who use either (and/or both). Kaplan was thorough and also great for this, but KA covered some very different things which were very VERY important. TPR I only read briefly - but have seen students ONLY use this resource for great results as well.
At the end of the day - Are you learning the concepts, and are you able to answer the questions correct (especially on AAMC material?). Are you retaining things and are you able to understand why? That is what is critical for the MCAT. Whatever source that allows you to do that is great. If you're super confident in a subject - the resource you use for content review doesn't matter much, if you are less confident - you will have to fill in the gaps and no one resource (besides Berkeley for Chem/Phys/Ochem) will suddenly make you a master.
Not a bad idea to mix and match books. The way the book pricing works though is that buying 2 full sets (especially if you go used, and an edition older) is cheaper than buying individual books from most companies (at least for EK and Kaplan that is the case, not so much for Berkeley in my experience) in all marketplaces - so picking up two full sets might actually cost you something similar to 3 of the most sought after individual books from each book set. Definitely price shop for this. For example I've seen one Kaplan/EK book go for $30 bucks with tax - while a whole set of Kaplan and/or EK (latest editions) go for $80-120 each. For older editions the pricing is more like $15 for a single in great condition, and $50-60 for the set.
TLDR:
For classes you haven't taken before - get a second perspective before you start even practicing. Weather it be from videos or KA. Don't use one resource. For classes you have taken the classes before for and retained well - use any resource, and fill in the gaps with practice.
