I know Bigger is intended to take 3-3.5 hours, but how does doing it with 2 children change that? Other than the obvious separate math, LA, and one in the ERS and one in DITHOR4/5.
Where do I find info about what the assignments are and how to do them for the extension readings?
How do I guide her in her written narrations if we've not really gotten those skills learned yet (for either of us
)Do I alter the science lab sheet assignments for her in any way or just have her do them as assigned in the science box?
Since she is a 5th grader, should I have her do more of the reading on her own? Possibly doing some of the reading aloud for her and her brother? I don't want to alienate her by sending her to do work on her own since we are looking forward to learning together...but I do want her to have some sort independence.
She needs to be doing copywork, and I know for the most part she can finish copying the poem in one to two sittings. So, what should we do, if anything, to add copywork for the other days? She is a messy writer and needs practice in careful, correct writing. She writes either super neat and super slow, or super messy and super fast. Perhaps we should just have her do copywork over the summer prior to starting Bigger?
I know many moms use one core guide, and bump up the basics by using the right side of another guide. How is it I'm using Bigger with my 5th grader and not using the right side of another guide? LOL How did we arrive to that and is that right? Will using PHFHG with a 6th grader work without adding in the right side of CTC? I know Julie (my3sons) and I discussed my options for writing instruction already, but beyond writing, is just using the extensions and not using the right side of the next guide correct?
With two kids, do we have two separate time lines and two separate vocab boxes?
How much time do the extensions add from Bigger? And what of my time is required for the assignments for the extensions?
On a side note, I decided to reassure myself of combining by doing about 15-30 minutes of Bigger each evening with my kids now, to make sure my DS7 will be able to handle it. We did Reading About History and the activity last night...I let him play with an abacus (math guy
) while I read and talked about the places on the wall map as we read. He enjoyed it! He did great! He even gave a primitive narration to Dad about Columbus and the "sea of darkness". He knew the names of all the continents for the activity, and really enjoyed finding and tracing them. I think he's going to do wonderfully! He wants to do more tonight 
Since my DD9 already did the first 10 units or so of Bigger, I think I'll continue on with him as we are able at night, with her listening in while playing with the baby in the entryway (gated and used as play area) right beside the school room so he's gaining the skills and she's getting the review from when she did it before. Doing it this way means we may get two full days (not the basics of course since he's doing that in school) each week, so we may get about 6-8 units done before they are out for the summer. Then maybe we'll a little each day or two over the summer to get our groove going and pick up strong in August. I want to be ready to move forward come August since we'll only have 4 days per week for school, so I really like the idea of training him in these skills very slowly now. He also enjoys the time together, which is far different than the time we "spend together" doing homework from public school. We will of course, be starting him on Unit 1 Day 1 in August for him in grammar, cursive, spelling, and math activities. I think this will work out fine... ??
Also the appendix of Bigger Hearts has detailed directions for studied dictations and oral narrations, so no sweat even if you have no previous experience. The Extension schedule and follow-up suggestions are also in the appendix, and extension books are meant to be read independently by your child. They might add about another 20 to 30 minutes of work for the older child. Your older child could certainly do the science box independently, to give her more challenge. If you are using the HOD recommendations for language arts and math, you could get the next guide up (Preparing) to help with scheduling and to provide higher level dictation passages, since the appendix of Bigger contains a schedule for Singapore Math 3A and 3B, and dictation passages are "level 2". Preparing has dictation levels 2,3, and 4 and a schedule for Singapore 2,3, and 4 in the appendix, and the daily lessons schedule Rod & Staff Grammar 3 and 4. CTC has R and S Grammar 4 and 5 in the daily plans, dictation levels 3, 4, and 5 and a schedule for Rod and Staff 6 in the appendix, along with schedules for Singapore 3, 4, and 5. You should have each child make their own timeline and vocabulary box. I think the copywork is sufficient in Bigger, but if you felt the need to, you could add a handwriting program like Cheerful Cursive or the Italic Handwriting.
If your older child knows cursive and jusgt needs to work on practice, then yes, using the poetry for copywork and having her do some of her other writing in cursive should be fine.