1blessedmom,
The ladies are doing a great job of discussing possible options with you!

As you think through what would be the best fit for your two kiddos, I'll encourage you to do a few things that will really help make it clearer as to where they fit best.
First, put aside any thoughts of which history cycle your students have covered, except for perhaps your high school student only if it certain history requirements must be met for credit. Next, set aside any ponderings about which science texts your students may have already used. Last, place your students as individuals, putting aside any thoughts of combining.

Use the first page only of the placement chart to place each student.
If you do these things, you will truly see where each child fits best and in which program. Correct placement in HOD is based on the skills shown on the first page of the placement chart, because those skills are required to complete all areas of the guide. So, if you place a child based on the history cycle or on the science program, you may have an inaccurate placement, making all of the rest of the guide a poor fit. This would be compounded in the years to come, as one guide prepares a child very well for the next guide in line!
So with that in mind, here is a link to the placement chart:
http://www.heartofdakota.com/placing-your-child.php
If you get a chance to pop back and share where each of your students fit as individuals on the first page only of the chart, then we can help you fine-tune their placement!

I know this will feel different than the approach used for placement found in other homeschool companies, but you'll find the reason for this to be that at HOD the skills are very important in placement. This is because CM type skills build from guide to guide (as well as the difficulty of the material and how the skills are applied). Even if you end up reusing a science text or a history text that you may have already read, you'll find that when the text is done in the CM style used in our guides (and likely is read independently by the student), it will feel completely different!

We have had this experience ourselves with our own oldest son, as have other moms on the board who have repeated a text. My oldest son even had to repeat a history cycle he'd just done when we switched to a more CM-style curriculum. It still worked well, and he gained so much!
We look forward to hearing back from you when you get a chance.
Blessings,
Carrie