What a wealth of wisdom there is here in water2wine's post! Her concrete examples of how you can look at your dc individually, and then determine which pacing is best, while also considering how to combine, makes total sense! I agree whole-heartedly with every single thing she said in her well-thought-out post, and I think when you get a chance to read it, it will give you an excellent direction to take with your 2 dc.

A big factor in this whole scenario is how your dd develops in her LA skills, which is why I think you have an excellent idea here in what you said:
alydar wrote:...I'm thinking now that I may order the kindergarten skills books, another set of the earlybird math, and TRL and just start adding that in to her day. Basically, I'll be bumping her up officially to kindy level in LHFHG...
I think this will answer your question about whether you can combine your dd and ds. By starting these LA skills with your dd, you will be able to see how she does with them. Some dc just take off, and others need more time to develop. The result of both can still be excellent readers and writers, so I think in this endeavor it will be important not to push dd in the hopes of making combining happen, but instead of just looking at this like a way to begin these LA skills with her at a pace she can handle and blossom with, and if it just so happens she takes off, then I do believe combining your 2 dc is definitely doable!

If you are finding dd is blossoming, and it is worth perhaps slowing ds down so she can join him, then I would do that. If you are finding frustration, tears, you are having to drop writing skills, etc. from the guide so she can do the guide, then that is a sign to you that not combining would be better for all around.
This portion of water2wine's post was especially helpful in thinking through this...
water2wine wrote:...The best advice I could give is to look hard at their learning potential and the skills they may lack on the placement chart and honestly ask yourself at the speed they learn will they be able to fill in the gaps or would moving them slower really be the better option in the long run. I think the taking the one side of one guide where they fit and matching it to the other side of the guide where they fit there can really be a great solution.

It's a really great solution I think for a child that moves forward in a kind of predictable fashion. The beauty of HOD is that you can meet them at their needs.

I think however you do that though it is wise to try to make sure you keep that wonderful thing of being able to meet them at their level. In other words try not to move them beyond the ability of the guides to in a reasonable way meet them at their level. Sometimes you have to just try and be open a bit in adjusting as you go if you are a little off target in an area.
It helps me a lot to look at the guide above and see the skills that are needed so that all year I can be moving them towards filling in those gaps a bit. You may not be able to predict completely but to have an idea of where you are headed really helps. 
I bolded the portion that resonates with me especially. This is the single best indicator for me if my dc are going to thrive in the next guide - they need to be able to do ALL of the current guide they are in by its conclusion. I hope this helps, but I really like your plan to start her on the K LA in LHFHG and see how it goes! Oh, and if you get going and find as your ds finishes LHFHG that dd needs LHFHG again, then I'd just do LHFHG again, but do Beyond's Storytime with the 2 of them instead of doing the LHFHG Storytime with her, as Beyond's Storytime will be new for her and be fine for her to do with him. Also, you can do the Gr. 1 LHFHG science to switch the science up some, and doing the 1 year sweep of history again will be fine, as it is varied enough to still be interesting - there is so much there to enjoy - and Bible stories (though just a portion of LHFHG) are meant to be enjoyed again and again!

I'd just choose the other LHFHG Bible, whichever one you didn't already use, and use that.

HTH!
In Christ,
Julie