Welcome to the HOD Board! I know more moms will respond here given time - we all help one another find a good placement. I am wondering how strongly you feel about combining? Every family has a different feeling about that, and sometimes it's easier to combine, and sometimes it's easier just to do 2 separate programs and combine down the road a bit.
If you want to combine, I am leaning toward Bigger Hearts for His Glory (BHFHG). It meets in the middle for your 2 dc better. We did BHFHG last year, and are now doing Preparing Hearts for His Glory (PHFHG). I cannot tell you how many times I have been thankful we did BHFHG first; it just did an excellent job of getting my ds ready to do PHFHG well. The Emerging Reader's Schedule is in the Appendix of BHFHG and so is spelling lists and also beginning dictation. BHFHG also teaches how to do the vocabulary cards, oral narration, timeline, geography, experiments, Bible discussions/memory work, notebooking, copywork - all on a much more introductory level. Then in PHFHG, these skills are meant to be much more independent, and other skills are added to them - like written narrations, cursive writing, research questions, independent science questions and lab experiments, more independent Bible study, longer Bible passages to memorize, 2 levels of dictation, etc.
I would love to get to know more about you and your dc though to feel comfortable giving you advice.

So, I have a little favor to ask! Could you please look at the placement chart, paying the most attention to the first page of it? If you could look at your dc separately, and place them where you think they really should go on their own, and let us know - that would be great - even if you don't end up going with that, it will help with placement.
http://www.heartofdakota.com/placing-your-child.php
Also, could you give a tiny bit of info for each child about where they are with writing, grammar, and math? Will you be using the math / grammar, or are you using something else for those? If you are considering using Singapore math (which we do really enjoy), you may want to take a minute and have them take the placement test on that site (it's free and doesn't take long). Then, share those results too if you could please. (Just click on "placement" at the top of the screen at this site:)
www.singaporemath.com
I want to really encourage you that HOD would work wonderfully well in your home!

I think it is important to spend time on placement - so you are wise to ask for advice. I know many other curriculums out there give a fast answer (that is often the same from family to family, meaning they do not consider each family's needs). HOD believes in making sure you get the best fit possible from the start. So, let us help you out here by giving us a little bit more of this information - and we will make sure you get the best fit for your family - it is, after all, a whole YEAR of your life - it's important we get it right!
In Christ,
Julie