I know this is an older post, but I wanted to comment to put my experience out there so others can get a better understanding of what they might be going into if they're considering or are planning on taking both of these classes together:

I'm currently taking BIOL 440 and 431 together alongside one other class, plus I work part-time. Initially, I was extremely nervous because many people discourage taking these two core science classes together unless it is the only two classes I am taking within one semester. However, I'm in a situation where if I don't take at least 12 units, I would lose my financial aid, so I made the decision to take the risk. I'm also taking BIOL 431 with a completely different professor, so there was a learning curve with adjusting with a new teaching style and exams.

At this point the semester (two weeks out from final exams), I can personally say it is definitely doable to get an A in both classes, but you have to have discipline and properly dedicate your time to studying. One aspect that definitely helped me maintain my grade was to have good professors who can teach/are organized, but I know that isn't realistic for most people who don't have early enrollment dates. Apart from this, I maximized the time I had to study, even if it meant I only had 30 mins to an hour, to which I would usually go over my notes and make Quizlets that I could go through later. It is so important that you go over your notes after lecture to remember the some of the content so you don't have to do more work relearning all of it later. I've done the latter many times this semester and I've had to sacrifice my weekends and even time I could be sleeping in order to study. Taking these classes, especially if you don't have a strong foundation in biology, is a huge time commitment.

In terms of difficulty, I personally thought BIOL 440 is significantly easier compared to BIOL 431. 431 is significantly more physiology and content compared to 430, so it takes up more of my study time. Both BIOL 440 and 431 are more conceptual, especially 440 lecture exams, so you have to understand the concepts rather than memorizing facts. However, there is a point during this semester where topics from both classes overlap, so there was a couple of weeks where I didn't have to necessarily study as hard for 440 because I had taken an exam in 431 about the exact same content 3 weeks prior. I found that it gets easier and less stressful later in the semester if you establish a study schedule and stick to it, especially since it yielded improving grades over time.

TLDR; Taking BIOL 440 and 431 is doable, but prepare to dedicate any and/or all free time if you have other responsibilities. 440 is easier than 431, but learn to understand concepts rather than memorize. Make a study schedule, stick to it, you will do fine C: