I was in your shoes with Allison and Garret 2 years ago.
I split them, and it was perfect for us!
Here's a little background on where we were leading up to that point...
Allison (age 4) & Garret (age 2) did Little Hands to Heaven together. We had a fabulous year!!!
Allison (age 5) & Garret (age 3)...I pondered what to do for quite a while. Finally decided to do a "half-speed" version of Little Hearts for His Glory with them together. Allison did Fine Motor Skills box and math and phonics daily though. It worked beautifully!!
Garret was a tag along on the math, but not the writing part of it - he is my MATH lover still today.
We ended up here because I needed something "school-like" to do with Garret. We were done with LHTH, and while I was pondering other curriculums for him, anything else was going to be more work for me. I considered stalling Allison for a year to let him be more ready for LHFHG, but didn't think that was the best choice either. So we did the s-l-o-w version of LHFHG with normal basics for her. This let me see if they would be a good match for the long haul. It became obvious over the year that she was ready to fly, and he was a typical almost-4 year old boy (who loved school as long as I wasn't going to drown him in it). Sooooo, in March of that year I let Allison take off full-speed on LHFHG and restarted LHTH with Garret. They still talk fondly of when they did "2-person school".
It was PERFECT for our family that year in our life.
Allison (age 6) & Garret (age 4)...Allison finished LHFHG and zoomed on into Beyond. Garret did LHTH. The split was made and both got exactly what they needed. Now, at that point Garret already knew his numbers, letters, shapes, colors, etc. But we still did LHTH as written. There is really so much more there than just those things. It is written right to that developmental level regardless of the "academic" knowledge that they have. I'd lean towards that guide for your younger gal if you split them. You could always add in phonics when she's ready. We did that mid-year with Garret when he was 4.5.
Allison and Garret still love to be involved in each other's learning, too, which is easy to do with the projects that are written throughout the guides. For example, on Friday in our house, Grant was experimenting with his car that he made in science - so all the kids (except Quinn, who was napping) went outside with him to time it and watch it race. Allison read about William Penn in history, so right after lunch we had Indian jumping games that we all participated in. (And my husband found out that 8 weeks after major abdominal surgery you still really don't have the muscles needed to JUMP, so he took over marking our jumps with masking tape
) Garret was also reading about the 1st Americans in history, and while Grant and I went to feed calves in the afternoon on the dairy, he and Allison worked on his Indian village together, complete with a tin foil river. I let the kids choose if they want siblings to jump in for the "fun" things that pop up, and it works wonderfully! Now, if it's a time consuming project, Grant usually can't jump in with the younger ones as he has plenty of work to complete for his own day, but he does the little things when they're ready to be done.
Have you read these posts by Carrie about HOD's philosophy and how combining can work well or not work well in different homes and situations?
viewtopic.php?f=6&t=9344 There's not one right answer...
Kathleen