Just wanted to pop in and asking how things are going and to see what you have decided to do.
As Julie mentioned, "when independence is the goal, it is wise to have children do the easier level of the 2 guide choices you are choosing between for them." I have found this to be absolutely true. My middle child is 20 months older than his younger sister and only 18 months younger than his big sister but you will notice that he is paired with the younger sibling. And because of his maturity and abilities, I technically could have had him in his own guide. But I do not have time to run three guides. I work at least 30 hours a week (but hoping to be able to cut way back by next school year) and if I am not working, I am at physical therapy or doctors appointments. When I am not injured, I love doing obstacle course races so I am working toward that goal (yes, I am saying I actually schedule me time for myself and to keep sane, I need it!) No matter what, life is insane. There was a time when I did run three guides and it was great. When I first started HOD, I started using Beyond for my younger two kids and Preparing for my oldest. I didn't fully understand why the ages were so important and my youngest was 5 when she did Beyond. It was a wonderful year. Our days got done quickly. No one was stressed. Everyone enjoyed what they were learning. The following year I decided to split up my younger two kids. My youngest wasn't in the age range for Bigger so bumped her down to Little Hearts, my son moved to Bigger and my oldest moved to CtC. It was a fantastic year! My kids learned a ton. My youngest is very advanced so she completed a lot of the reading from Little hearts on her own (especially the storytime books....those were her favorite

) I found that my son needed a lot of one on one to help him with the activities and I was glad to be of help. My oldest was fairly independent but she still needed some help for the first half of the year. However, she got much more independent as the year went on. The next year, I bumped my youngest to Bigger, my son to Preparing and my oldest to RtR. We made it 11 weeks in, I was working like crazy at this point, and I no longer had time to run 3 levels because my younger two kids required too much of my time. I dropped all of Preparing except for the science with my son and made him repeat Bigger and added in the extensions. It was the best thing I could have done. I no longer had to help him with so much. I no longer had so much to read out loud to the younger 2 kids. I had more time to spend with my oldest if she needed it. And my youngest was already doing the science box alone in Bigger because she could. For me, going down a guide was the right thing to do. My son had plenty of work to do in Bigger because I added the extensions for him and my youngest was perfectly challenged. You could definitely split your two youngest kids up. But you could just as easily keep them together and do the lower guide with both of them. I felt like Bigger really trained my kids on how to do Preparing well.
Many of the elements in science scheduled in Preparing are learned in Bigger. But once kids get to Preparing, they start doing them independently. And you can definitely have your kids do Bigger more independently too. I know time is a premium and you don't have time add in more work for yourself. Your older son could read the history and science out loud to your younger son. You could get books on CD for the story time. Your older son would do the extensions and he would then need to write a 3-5 sentence written narration on the reading each week. The other nice thing about Bigger vs Preparing is the younger child would get a bit more hand holding on the notebooking pages and science in general. You wrote you were skipping science because your son needed too much help. I think the science in Bigger really helps the kids prepare for the science in Preparing. Plus, I think this is probably my kids favorite subject from Bigger. I loved those books as well. But doing Bigger would definitely help prepare your younger child to tackle those things now and learn to do them before they are assigned independently in the next guide. Finally, by doing a year of Bigger, it would give you the opportunity to delay doing CtC for one more year. You mentioned that your youngest wasn't be ready fro CtC next year. So this would give both boys a full load for 2 years to prepare for the full load of CtC.
Definitely a lot to think about. And I was wondering what you decided to do.