Ah another kindred spirit! Over-planners of the year unite.

I, too, love to plan ahead. It's embarassing how many different long-term plans I have made through the years!
That being said, I do know where you're coming from and can try to help you out a little.
After Preparing Hearts for His Glory, the next 4 guides will do a 4 year cycle of world history. Year 1 of that cycle will be Ancient History (Egypt, Greece, and Rome). Year 2 of that cycle will be Middle Ages, Renaissance, and Reformation. Year 3 of that cycle will pick up where the Renaissance/Reformation left off and go to approximately the 1800's. Year 4 of the cycle will be 1800's to present day.
Currently the plan is for each guide in the cycle to gradually increase the student's level of independence a little more, until much of the student's day will be fairly independent. Daily teaching time will be reserved for those things that require deeper thought, application, and discussion.
At this point the plan is to rotate teaching time by quarters through the various areas of emphasis (i.e. history, science, Bible, geography). This would allow the teacher time to delve deeply into a subject area for a quarter and then give a fresh feel with a new area of emphasis the next 9 week period. Of course, the student will still be working on all areas throughout the year, but the teacher time would be rotating by quarter. All of this is only in the general planning stages, so I hesitate to post it (and reserve the right to change it

), but I do love to toss ideas out and get feedback on them, so we'll put it out there.
In our future guides, some teaching time will also still be required daily or weekly for those subjects that don't lend themselves well to more independent learning (i.e. grammar, creative writing, some parts of math and literature study).
I say all of this to show you that with the design of our guides to come it will not be difficult to do more than one of our programs at once. We wish to enable homeschool families to spend the necessary time needed to school their little ones while their older ones are able to work much more independently. Often, we find that in keeping up with their older kiddos families have no time to devote to the little ones coming up. That is not to say that we will neglect needed time with the older ones, it will just be structured differently and reserved for key areas that most need instruction.
After using our programs for the younger years, students will certainly feel like they have had plenty of time spent with you which lays the foundation for future more independent work later.
I hope that helps a little.
Blessings,
Carrie