You can easily combine with HOD, just like you can with other curriculums, but combining is not always what's best for dc or for moms.
I'm going to say this because I wish I had heard it when I first began homeschooling, as all I ever heard was I would need to combine my dc for homeschooling to work because it was "easier", and otherwise I'd "burn out". I wish I'd heard that combining works very well in some situations, while not combining works very well in some other situations.
When dc are farther apart in ability and maturity, it just makes good sense not to combine, as they have totally different needs from each other. Let's say for instance that a younger child is not reading and writing well, but an older child is reading chapter books and writing paragraphs. Charlotte Mason would say that older child should be moved toward reading their own material in all subject areas (CM said roughly around the age 9), as this promotes better retention and narration skills. If a 9 yo child is ready to fly with this type of independence but is paired with a younger sibling not yet close to reading, this forces the parent to be reading everything out loud to both of them in order for them to be combined. This makes combining take longer than teaching separate programs would take, and it prevents the older child from doing skills they should be doing for their age.
Likewise, if an older child is writing well and ready for written narration instruction (CM said roughly around the age 10), but is paired with a younger sibling not writing, the parent is forced to find something else for the younger child not yet writing to do (often something that is a skill far below this skill that is independent, such as coloring, as they cannot read or write yet). There is nothing wrong with coloring, but then at what point does younger child receive that excellent guided written narration instruction that the older child received? Often they don't, as the older child is always on to the next harder skill, while the younger is still just doing something to "tread water" while the older child finishes. Often the gap widens, and the parent is forced to continue to read everything aloud as the younger child cannot read independently yet. Or, because the older child has continued to move up in guides, even by the time the younger child can read, the reading is so incredibly difficult, that child still cannot read the material - which means either the parent is still reading aloud to the duo (again the older student missing vital independent skills) or the need to separate the two becomes so very obvious at this point as the mom is all but hoarse from reading pages aloud that were never meant to be read aloud by the parent. This can set up a precedence to be teaching to the older child's needs instead of the younger child's needs, and over time the younger child will just need something different. You can see how, because older child never did get to read the material on his own, the parent never did have time freed up to work with the younger child, who really probably needed more instruction earlier on.
Now imagine 2 dc who are quite close in ability. The younger is doing phonics and writing individual letters, while the older is just starting to read 3 letter words and is just beginning to write 3 letter words. Combining these dc together makes much sense! They will both require the parent to read the learning material aloud, and they will both require time to grow into doing reading and writing independently. There is no reason they cannot both do the same history, science, poetry, Bible reading, etc., and while they may need separate reading and math instruction, this is fairly easy to accomplish. LIkewise, imagine older dc, who are both reading and writing fairly well, one who is reading chapter books, and one who is reading longer chapter books. HOD makes it easy to combine these dc as the younger child can do the program as is, and the older child can do the extensions. If the older child is writing pages, and the younger child is writing paragraphs, both can easily receive instruction on written narration practice, with one completing more than the other.
Combining is a winning situation here, as long as the younger child is not being asked to listen to material that is too mature for her ears.
When combining, it's important not to fool ourselves into thinking it will automatically be easier because it is 1 program to teach vs. several programs to teach. Sometimes just finishing the 4 yo's school in 30 minutes, and finishing the 6 yo's school in 2 1/2 hours because that's where they place best, is super easy compared to trying to slow things down so the 4 yo can catch up to the 6 yo, or trying to rush along 4 yo to catch up to the 6 yo. Other times, combining is the perfect answer as dc are close in abilities, work well together, and place in similar guides anyway. I will say either way, the single biggest factor in making homeschooling multiple dc easier is when dc reach the age of being able to read material on their own - both directions in guides and materials in living books. The next biggest factor would be when they are writing fairly well on their own. It is necessary for dc to move toward this independence in their learning as along with it comes age appropriate skills. I hope that helps as you consider what's best for your dc - whether it be combining or not combining.
ETA: Outside circumstances also can play a part in whether or not to consider combining. For example, working outside the home many hours, dc with health concerns where many doctor's appointments are necessary, being in poor health yourself, and having a very large family and being stretched thin, etc. Thinking back to the wonderful ladies I've met at book fairs, these are also important things to consider when choosing whether to combine or not.