Allison,
In this situation, it really depends on how independent your older son is? My oldest is very independent and strong-willed and very much enjoys being in charge of his learning (with me in the helper role).
The more of his day he could take over on his own, the happier he was. So, if this is the case with your older son, you could use the first 9 weeks of the school year to "train him" to use the Preparing guide very independently. You could be doing "Bigger..." and "Little Hearts..." at half-speed during this "training period". After the training is over, bump "Bigger Hearts..." up to full-speed, still keeping "Little Hearts..." at half-speed. Once you hit your stride with "Preparing..." and "Bigger....", then bump up "Little Hearts..." to full-speed.
During the "training period", require a very high standard of work from your oldest. Set the timer and keep him on schedule. Schedule the places where he is to do his work when. Have him check off his items and hand them all in. Go over the directions in each box with him as you check his work to make sure he followed them all. This will teach him to read directions carefully.
In Preparing..., you would still be scheduling some time to do the questioning and discussions scheduled in the Reading About History box (but he could read the readings himself), be doing the Bible discussion of the Psalms scheduled on Days 1-2 (he'd be independent on Days 3-4), teaching the poetry lesson, and most likely doing the storytime read-alouds. But, the rest of the guide can really be done independently.
I do still teach the grammar lesson to my oldest also, and dictate his passages for spelling, and we do still do the DITHR discussions together too on alternating days as scheduled in DITHR.
Just some food for thought.
Blessings,
Carrie