Hi, Katrina! Welcome to the board. This is my first year w/ HOD- so I only have limited amounts of knowledge...

If you really want to do just one guide and your 1st grader fits more in Beyond, I think it is usually suggested that you actually combine with the younger in mind NOT the older. I know it seems strange because we always get so concerned about our oldest not being challenged enough. If you go with Beyond for both, it would be easier to beef up the 3R's for your 2nd grader than to have to water down for your 1st grader because Bigger is a lot of work and a lot of oral listening w/o pictures, narrating, copywork, etc. (for a 1st grader- I think). You would have to really change it to help your younger and if he is not ready for a lot of writing, you would be changing Bigger a lot for him or possibly be dealing with meltdowns. It just sounds like it might be too much for your younger! Maybe other more experienced ladies can give more expert advice.
I know that last year, when I was considering combining my 4th grader and my 1st grader, Carrie suggested that I combine them in Beyond and for the Left side of the manual (the History/Bible side) and then beef up the Rt. side for my 4th grader-(the 3R's) and I have been informed that the history is plenty "meaty" for even the big brothers! We actually ended up keeping our 1st grader in ps and bringing home just our 4th grader and so we did Bigger for him w/o extensions and it has been plenty "meaty" for him. Next year, we will bring home our then 2nd grade ds and put him in Beyond and he is very bright and an academic over achiever! I just do not want to teach a guide back to back.
Now, if you want to teach 2 guides, I've heard many ladies say that it's not difficult because HOD is so wonderful!( I will find this out next year!)You could do Bigger w/ your 2nd grader and Beyond w/ your 1st grader. The only "problem" that I could potentially see (if you call it that) would be teaching Bigger 2 years in a row which I've heard can be difficult on a mom. Another idea...??? Have you looked at LHFHG for your 1st grader? You could use the 1st grade plans for that if wanted another idea to put some space in between guides.
As for the history readings in Beyond- I do not think that you have to have them read to understand the history in Bigger. They are 2 different approaches to American History. The biographical approach in Bigger has you learning about history while you are learning about some of the amazing players in the history of America. I can't really talk much about Beyond because we haven't done it yet. (some of the other ladies may suggest you read them). I know it's hard to not want to miss out on anything. As for the Grammar/language arts part of the curriculum, YEs, you could set aside and just do the copywork and narration as it is suggested in the guide and then do R& S 2 if you decide to do Bigger for your ds.
I hope this helps soem. Pop in again and let us know what you think- The ladies here are so encouraging and helpful- you will love HOD whatever you decide. You know your dc best!
Dawn