Kim,
It can be intimidating to try to teach a child to read while trying to help another child gain needed skills too, so please know that just because you don't feel like you can't do two guides right now doesn't mean you will always feel this way (or be at this stage in life).
Keeping what you have shared so far in mind, if you feel that you must combine I would encourage you to actually combine your older with your younger child in LHFHG (rather than attempting to push your 5 year old forward too early in Beyond or Bigger). This is because you will be stealing your thunder by allowing your younger child to listen to all of the readings in Beyond or Bigger when he or she is unable to gain any of the needed skills that go along with those readings.
Here's a link that explains this more fully:
viewtopic.php?f=6&t=9341
I know that your older child has likely been through LHFHG (and also been through much of Beyond), however I still feel that if you deeply desire to combine you could have your older child just join your younger for much of LHFHG.

Your older does fall within the age range for LHFHG (and since you have the Beyond manual) you could just continue to use the 3R's from the right side of the Beyond guide for your 7 year old.
After completing LHFHG with your pair, they could head into Beyond together (and you could borrow the 3R's from Bigger for your older child as needed).
There is a list in the back of LHFHG of possible books to use to extend the learning in LHFHG if desired. While this exact list of titles isn't needed, it would give you some ideas of possible topics you could add for your 7 year old. I wouldn't get too worried about extending LHFHG though, as you don't want that to take over your life either.
Just make sure that your 7 year old's 3 Rs are where they should be, and this combining scenario would work (especially if your 7 year old appears to be enjoying joining in with your younger child, and you have taken a whole semester off from Beyond at this point already).

Honestly, sometimes the best placement is the one that gets done in the day-to-day.
One other benefit of this plan is that it always leaves you the option of jumping your older child forward if you ever felt you were ready to handle more than one guide, yet it doesn't leave you little one always trailing behind.
Blessings,
Carrie