Exactly. Plus, it can be more than just those smaller muscles getting hit twice a week.
I am a "Doggcrapp Guy", and Dante is very big on overlapping exercises to bring up lagging bodyparts. For instance....
You mentioned Tri's getting hit twice a week...once directly on their own, and once indirectly on chest day.
Well, it all depends on what exercises you use on your tricep day and you could very easily work some extra stuff in to help bring your chest up if it is lagging.
Now, lets say you do 3 tricep exercises, and they are skullcrushers, pushdowns, and kickbacks. Are you getting any extra chest work? Definitely not.
However, lets say for your 3 tricep exercises, you decide to use reverse grip bench presses, dumbbell floor presses and dips. These are all generally known as 'tricep' movements, but obviously all are going to overlap and hit your chest pretty good as well. Not enough that it's going to blow your chest out completely and cut into your recovery greatly as if you did a whole extra chest day, but definitely enough to get you some good extra work in there, up your capacity to handle work in that muscle group and give it some extra volume to help bring it up to your stronger bodyparts. The same principles can be applied for basically any bodypart. It just depends on what you want to prioritize at the time.
Also, re: splitting up weekly volume, that is a HUGE debate in the bodybuilding world now and I'm sure will be for years to come. There are guys who are definitely on the volume/trash a muscle side, like Professor X, then there are guys on the frequency side, like Disc Hoss, and you'll find coaches who advocate both. I think a lot of it is just trying both and seeing what works for you. Some guys will hit the same muscle group 3-4 times a week, some 2, and some 1 and just blow it out.
I'll say this- 2 is generally a safe bet for everyone it seems, and a good middle of the road approach. The others are going to be more based on how your body responds specifically. I, personally, do MUCH better and grow faster with higher frequency and lower volume than I do on low frequency and high volume, but there are tons of other guys who are the opposite. There really are no cookie cutter answers for it, but if you play around with each for 6-8 week cycles, you can get a pretty good idea, provided the workouts are all logical and your diet is in order, of which path YOU should be following.
For a tremendous learning experience on this subject,I suggest everyone should take a look at Christian Thibaudeau's Training Strategy Handbook. One of, if not the best, articles ever written on this website.