We do use 4 guides at our house. We're currently using LHTH, Bigger Hearts, RTR, and have a 9th grader piloting much of our geography guide along with his other high school level work. With that being said though, we typically don't recommend doing two more teacher intensive guides like Bigger Hearts and Preparing Hearts at the same time, as it gets to be too much for the teacher.
So with that thinking, it would be far better for you to do 3 guides, with one of them being LHTH. I think this would really help keep your year in a better balance. I'm also thinking from what you've shared so far that Preparing Hearts will be too much for your sweet child from Liberia. It actually sounds like she would fit better in Bigger Hearts overall, unless I've gotten the wrong picture so far. I can see the qaundry of having your older child doing the same program as a much younger sibling, so I wonder if you could use the Extension Pack with your older child and just read it to her. In that way she would see that she has more school for her older level than her younger sister, and even though you'd read aloud the Extension Pack to her it would be less than the reading you'd be doing aloud for her if you added a whole program such as Preparing Hearts to your day.
If you didn't want your girls to compare themselves so much, you could consider doing just the science from Preparing Hearts for your older daughter to give her something different from what the younger was doing in that area too. That would add less time to your day but give her something of her own.
Another thought I had was that you could also place your older child in a higher level of DITHR, keeping your 2nd grader in Level 2/3 and placing your older child in Level 4/5. Even if your older student can't do the writing for her Student Book, you could just take dictation from her and act as her scribe, writing her thoughts for her. This would allow her to have a different, older level of the Student Book and keep the comparisons less there too.
Anyway, then they would then only be combined for the left side of the Bigger Heart's guide each day and for Storytime. These areas combine easily across wider age spans, but they would have much of their own tailored learning in the 3R's and in science.
Another option I'd pondered for your unique situation would be to place your 2nd grader in Bigger Hearts and your older child in Preparing Hearts, but do both programs at half-speed, taking two years to complete each one. This would keep your teaching time more manageable and would help the kiddos be more separate. However, it would take a bit more juggling each day though, and may make you lose some of the open-and-go aspect of the teaching though.