Hi Tina! I have a ds who would have loved to have read textbooks, filled out an accompanying worksheet, and been done with it.

He is my oldest and a perfectionist. He is very bright and not lazy, but he tends to prefer there be one answer to things, and he loves to give that one answer and be "right".

I have chosen to encourage him to expand his horizons by doing the living books in HOD, and I've been very happy with the results. He has to think so much more when questions are higher level, when assignments are open-ended, for those types of questions/assignments are more complex, and they don't have the one, right answer. Likewise, living books are more complex than textbooks, as they weave many different layers throughout the book. For instance, an author's personality or unique writing style is inherent, whereas a textbook reads like a textbook, and the author is rarely a consideration when choosing it. Christian beliefs are often a part of many of the living books HOD chooses, and that is of course most often left out of textbooks. Over time, I'd be willing to bet your ds would be like mine, and would gravitate more and more to living books, rereading, and rereading them, just for fun, over and over again.

These are just a few of the many reasons your bright ds should be challenged to move out of his comfort zone and read living books (IMO).

On a side note, having a child like this can be a good thing character-wise, as they tend to know right from wrong and adhere to those beliefs strictly. The challenge is to also teach them compassion for others, even when they make a mistake, as dc like this tend to be somewhat judgmental. This has been our experience at least.
So, in answer to your questions

- You are already being able to be very economical by combining 2 dc in one program, and very little extra will be needed for your older ds other than the extension package, which extends the areas of history and science, and whatever LA/math levels he needs. In BHFHG, science becomes a daily subject that has a variety of skills and follow-ups throughout the week:
*Day 1: practice narration by retelling the science story
*Day 2: create a science notebook entry
*Day 3: conduct an experiment related to the reading and log it
in a science notebook
*Day 4: practice narration by retelling the science story
*Day 5: conduct an experiment related to the reading and orally
discuss it
The BHFHG Appendix explains the extensions well. Here is a portion of what it says...
Older students should first listen along with younger students to the scheduled history readings in Bigger Hearts for His Glory. Then, older students will independently read the assigned text and literature from the Extension Package for each unit. These readings will provide more details about people and events already being studied as well as immersing older students in the time period being studied.
It goes on to say this about science...
In the area of science, Exploring Planet Earth is used to provide more advanced science material on topics not already covered in the science portion of Bigger Hearts for His Glory. This text was not chosen to use as a substitute for the science in Bigger Hearts for His Glory, rather it was selected to use in addition to the science resources and experiments already found in Bigger Hearts for His Glory. Exploring Planet Earth covers a similar time period in history to that being studied in Bigger Hearts for His Glory.
Blending a creationism perspective of science with definitions of terms and identification of famous explorers and scientists, this book gives students an excellent knowledge of people and places. Rather than corresponding by unit to the science plans, it is meant to be used as a stand alone text that introduces students to even more men of science in a biographical way. The Story of Thomas Alva Edison is scheduled in the Appendix in addition to the biography of Edison already used in Bigger Hearts for His Glory. It provides more events and details from Edison's life. .
Here are the follow-ups suggested for the extensions
Depending on your goals for your older students’ independent readings, you may want to assess their reading comprehension in some way. Some suggestions for assessment of your older students’ reading could include having them do a combination of the following things:
• orally retell what they’ve read (suggested twice during each unit)
• write a one paragraph summary of the reading (suggested twice during each unit)
• draw a picture with captions about the reading (suggested once during each unit)
So there is a nice variety of follow-ups that is more directed than journaling, and thus more skill focused, while still providing the opportunity (and the challenge) for dc to respond in more creative, personal ways to what they've read. As far as approaching your ds's learning style, I would give him a rich diet of CM style learning, and watch him blossom!
I remember Andreola commenting specifically about how it is important not to tailor learning toward personal learning styles, but rather to encourage development of them all, as this creates a well-rounded individual who can function well in a variety of situations, with a variety of people - which this world is sure full of.
I hope something here helps! Good questions!
In Christ,
Julie